Voltage detecting apparatus and line voltage detecting apparatus having a detection electrode disposed facing a detected object

ABSTRACT

A voltage detector that detects an AC voltage in an object includes: an electrode disposed facing the object; a current-to-voltage converter that has a first input set at a reference voltage and a second input connected to the electrode and converts a detection current, which corresponds to a potential difference between the detected AC voltage and the reference voltage on a path including the electrode and a feedback circuit connected to the second input, to a detection signal; an integrating circuit that integrates the detection signal and outputs an integrated signal whose amplitude changes in accordance with the potential difference; an insulating circuit that inputs the detection signal or the integrated signal, and outputs the signal so as to be electrically insulated from the input; and a voltage generating circuit that generates the reference voltage by amplifying a signal based on the integrated signal to reduce the potential difference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage detecting apparatus thatdetects the voltage of a detected object and to a line voltage detectingapparatus equipped with such voltage detecting apparatus.

2. Description of the Related Art

As one example of this type of a voltage detecting apparatus, anon-contact voltage measuring apparatus (hereinafter, also referred toas a “voltage detecting apparatus”) disclosed by Japanese ExaminedPatent Application Publication H07-58297 is known. This voltagedetecting apparatus is a non-contact potential measuring apparatus thatreads changes in potential of a detected object (or “sample”) in anon-contact manner, and includes a metal prong with a pointed end, afeedback circuit that detects a field emission current or tunnel currentvia the metal prong and applies a voltage to the metal prong so as tokeep such current constant, and a circuit that reads the voltage of themetal prong. With this voltage detecting apparatus, when the metal prongis held near the detected object, the feedback circuit carries outcontrol over the voltage applied to the metal prong so as to keep thefield emission current or tunnel current constant. Since the voltage ofthe metal prong at this time will follow the voltage of the detectedobject, by reading the voltage of the metal prong, it is possible toread (i.e., detect) changes in the voltage of the detected object. Thisvoltage detecting apparatus is constructed using a current-to-voltageconverter as part of the feedback circuit and converts the fieldemission current or tunnel current to a voltage signal using thecurrent-to-voltage converter.

SUMMARY OF THE INVENTION

However, by investigating the voltage detecting apparatus describedabove, the present inventor found the following problems with suchvoltage detecting apparatus. With this voltage detecting apparatus, themetal prong whose voltage changes in keeping with the voltage of thedetected object (or “sample”) is directly connected to an input terminalof the current-to-voltage converter. This means that as a first problem,to make it possible to use the voltage detecting apparatus on a detectedobject with a high voltage, it is necessary to construct thecurrent-to-voltage converter using an operational amplifier with a highwithstand voltage, that is, an electronic component with a highwithstand voltage, resulting in an increase in apparatus cost.

Also, in the voltage detecting apparatus described above, the metalprong to which the output of the feedback circuit is applied isconnected to the current-to-voltage converter and since the inputimpedance of a current-to-voltage converter is normally low, thefeedback circuit has a heavy load. Accordingly, as a second problemwhich should be solved, in the voltage detecting apparatus describedabove, due to the heavy load of the feedback circuit, although it willbe possible for the voltage applied to the metal prong to follow thevariations in the potential of the detected object when the change inpotential of the detected object is slow (i.e., when the period of thevariations is long), when the change in the potential of the detectedobject is fast (i.e., when the period of the variations is short), itwill be difficult for the voltage applied to the metal prong to followthe variations in the potential of the detected object.

The present inventor has already proposed another non-contact voltagedetecting apparatus (see Japanese Patent Application No. 2008-158754).This voltage detecting apparatus applies a voltage to a guard electrodeof a floating circuit unit that includes a detection electrode that iscapacitively-coupled to the detected object (or “measured object”). Bycarrying out feedback control to cause such voltage to follow thevoltage of the detected object, the voltage of the detected object isdetected.

However, by further investigating the voltage detecting apparatusesdescribed above, the present inventor found that even if the secondproblem described above is solved, since such voltage detectingapparatuses are constructed so as to use feedback control, when thevoltage of the detected object is an AC voltage (detected AC voltage) ofa high frequency, due to the performance of the circuit elements in thevoltage detecting apparatuses, the voltage generated by feedback controlwill not be able to reliably follow the voltage of the detected object,resulting in a problem (hereinafter “third problem”) which should besolved that it is difficult to correctly detect the detected AC voltage.

The present invention was conceived to solve the first problem describedabove and it is a first object of the present invention to provide a(non-contact) voltage detecting apparatus and a line voltage detectingapparatus that are capable of detecting the voltage of a detected objectwith a high voltage while avoiding the use of electronic components witha high withstand voltage.

The present invention was also conceived to solve the second problemdescribed above and it is a second object of the present invention toprovide a wideband voltage detecting apparatus and a line voltagedetecting apparatus that are capable of detecting a voltage of adetected object that changes quickly.

The present invention was also conceived to solve the third problemdescribed above and it is a third object of the present invention toprovide a non-contact voltage detecting apparatus and a line voltagedetecting apparatus that are capable of detecting a detected AC voltageacross a wider band.

To achieve the above first object, a first voltage detecting apparatusthat detects a detected AC voltage generated in a detected objectcomprises: a detection electrode that is disposed facing the detectedobject; a current-to-voltage converting circuit including an operationalamplifier that has a first input terminal set at a reference voltage anda second input terminal directly or indirectly connected to thedetection electrode and that converts a detection current, which flowswith a current value that corresponds to an AC potential differencebetween the detected AC voltage and the reference voltage on a path thatincludes the detection electrode and a feedback circuit connected to thesecond input terminal, to a detection voltage signal and outputs thedetection voltage signal; an integrating circuit that integrates thedetection voltage signal and outputs an integrated signal whoseamplitude changes in accordance with the potential difference; aninsulating circuit that is disposed so as to come one of before andafter the integrating circuit, inputs one out of the detection voltagesignal and the integrated signal, and outputs the inputted signal so asto be electrically insulated from an input side thereof; and a voltagegenerating circuit that amplifies a signal based on the integratedsignal so as to reduce the potential difference and thereby generatesthe reference signal.

With the first voltage detecting apparatus, in a state where thedetection electrode has been disposed facing the detected object, thecurrent-to-voltage converting unit converts the detection current, whichflows (via the capacitance formed between the detected object and thedetection electrode and the detection electrode on a path that includesthe detection electrode and the feedback circuit connected to the secondinput terminal) with a current value that corresponds to the potentialdifference between the detected AC voltage and the reference voltage toa detection voltage signal. The integrating circuit integrates thedetection voltage signal (more specifically, the current that flowsbased on the potential difference between the detection voltage signaland the reference voltage) to generate an integrated signal whoseamplitude changes in accordance with the potential difference betweenthe voltage of the detection electrode and the AC voltage of thedetected object. The insulating circuit that is disposed so as to comeone of before and after the integrating circuit inputs one out of thedetection voltage signal and the integrated signal and outputs so as tobe electrically insulated from the input side thereof. The voltagegenerating circuit amplifies a signal based on the integrated signal soas to reduce the potential difference between the detected AC voltageand the reference voltage and thereby generates the reference voltage.

According to the first voltage detecting apparatus, since the magnitudeof the capacitance is normally extremely small, it is possible to setthe impedance between the detected object and the detection electrode ata sufficiently high value (several MΩ) and as a result, it is possibleto set the input impedance of the current-to-voltage converting circuitat a relatively low value. This means that it becomes less likely thatan overvoltage will be applied to the current-to-voltage convertingcircuit, and even if a low-cost electronic component (such as anoperational amplifier) with a low input withstand voltage is used in thecurrent-to-voltage converting circuit, it will still be possible togreatly reduce the probability of breakdown of the current-to-voltageconverting circuit due to the potential difference between the detectedAC voltage and the reference voltage. Also, by adding a protectivecircuit such as a diode, it is possible to reliably avoid breakdown.

To achieve the above first object, another first voltage detectingapparatus that detects a detected AC voltage generated in a detectedobject comprises: a detection electrode that is disposed facing thedetected object; a current-to-voltage converting circuit including adetection unit that is disposed between the detection electrode and aposition of a reference voltage and converts a detection current, whichflows with a current value that corresponds to an AC potentialdifference between the detected AC voltage and the reference voltage, toa voltage signal and an amplifier that converts an impedance of thevoltage signal and outputs the converted voltage signal as a detectionvoltage signal; an integrating circuit that integrates the detectionvoltage signal and outputs an integrated signal whose amplitude changesin accordance with the potential difference; an insulating circuit thatis disposed so as to come one of before and after the integratingcircuit, inputs one out of the detection voltage signal and theintegrated signal, and outputs the inputted signal so as to beelectrically insulated from an input side thereof; and a voltagegenerating circuit that amplifies a signal based on the integratedsignal so as to reduce the potential difference and thereby generatesthe reference signal.

According to this other first voltage detecting apparatus, since it ispossible to set the input impedance of the current-to-voltage convertingcircuit at a relatively low value in the same way as in the firstvoltage detecting apparatus described above, it becomes less likely thatan overvoltage will be applied and it becomes possible to greatly reducethe probability of breakdown of the current-to-voltage convertingcircuit due to the potential difference between the detected AC voltageand the reference voltage.

To achieve the above first object, another first voltage detectingapparatus that detects a detected AC voltage generated in a detectedobject comprises: a detection electrode that is disposed facing thedetected object; an integrating circuit including an operationalamplifier that has a first input terminal set at a reference voltage anda second input terminal directly or indirectly connected to thedetection electrode and that integrates a detection current, which flowswith a current value that corresponds to an AC potential differencebetween the detected AC voltage and the reference voltage on a path thatincludes the detection electrode and a feedback circuit connected to thesecond input terminal and outputs the integrated signal whose amplitudechanges in accordance with the potential difference; an insulatingcircuit that inputs the integrated signal and outputs the inputtedintegrated signal so as to be electrically insulated from an input sidethereof; and a voltage generating circuit that amplifies a signal basedon the integrated signal so as to reduce the potential difference andthereby generates the reference signal.

According to this other first voltage detecting apparatus, since it ispossible to set the input impedance of the current-to-voltage convertingcircuit at a relatively low value in the same way as in the firstvoltage detecting apparatus described above, it becomes less likely thatan overvoltage will be applied and it becomes possible to greatly reducethe probability of breakdown of the integrating circuit due to thepotential difference between the detected AC voltage and the referencevoltage.

In the first voltage detecting apparatuses described above, theinsulating circuit may include an optical insulating element and/or atransformer, and output the integrated signal, which is an analogsignal, so as to be electrically insulated from the input side thereof.

By using this construction, it becomes easy to electrically insulate(separate) the current-to-voltage converting circuit and the followingcircuitry at an arbitrary position. Also, when an optical insulatingelement is used, since there are favorable frequency characteristicsover a wide range of frequencies, it is possible to precisely detect theAC voltage of the detected object over a wide range of frequencies. Whena transformer is used as the insulating circuit, since a transformernormally has favorable frequency characteristics in a higher frequencyband than an optical insulating element, when the frequency of the ACvoltage is limited to a high frequency, by using a transformer, it ispossible to precisely detect the AC voltage. When the insulating circuitis constructed by connecting an optical insulating element and atransformer in parallel, by having the optical insulating elementoperate mainly at low frequencies and the transformer operate mainly athigh frequencies, it is possible to provide the insulating circuit withfavorable frequency characteristics over a wide frequency band. As aresult, it is possible to detect the AC voltage of the detected objectprecisely over a significantly wider range of frequencies.

In the first voltage detecting apparatuses described above, theinsulating circuit may include a digital isolator, and output theintegrated signal, which is a digital signal, so as to be electricallyinsulated from the input side thereof.

By using this construction, it is possible to transmit a signal withhigh precision to the voltage generating circuit without being affectedby the temperature, changes over time, and the like of the transfer pathon which an electrically insulated signal is transferred. As a result,it is possible to significantly improve the detection precision of thedetected AC voltage.

The first voltage detecting apparatuses described above may furthercomprise a guard electrode that is set at the reference voltage andcover circuits from a circuit that comes after the detection electrodeto a primary-side circuit of the insulating circuit.

By using this construction, it is possible to make the circuits lesssusceptible to the effects of external magnetic fields, and as a result,it is possible to improve the detection precision of the detected ACvoltage.

In this case, an opening may be formed in the guard electrode and thedetection electrode may be disposed at a position inside the guardelectrode so as to face the opening in a state where the detectionelectrode does not protrude from the opening.

By disposing the detection electrode in a state where the detectionelectrode is inside the guard electrode at a position that faces anopening formed in the guard electrode without the detection electrodeprotruding from the opening (a “non-protruding state”), it is possibleto make the detection electrode less susceptible to the effects of theexternal magnetic fields. As a result, it is possible to significantlyimprove the detection precision of the detected AC voltage.

The first voltage detecting apparatuses described above may furthercomprise an insulator that covers an entire surface of the detectionelectrode that faces the detected object.

By using the above construction, it is possible to reliably preventshorting between the detected object and the detection electrode.

A first line voltage detecting apparatus comprises: a plurality of firstvoltage detecting apparatuses that are constructed so that the detectionelectrodes thereof are capable of being disposed facing a plurality ofpaths that correspond to the detected objects thereof and so as to becapable of detecting AC voltages generated on the respective paths asthe detected AC voltages thereof; and a calculation unit that calculatesa voltage difference between the AC voltages on two of the pathsdetected by a pair of first voltage detecting apparatuses out of theplurality of first voltage detecting apparatuses and thereby finds aline voltage between the two paths.

According to the first line voltage detecting apparatus, by using thefirst voltage detecting apparatus described above, even if low-costelectronic components (such as operational amplifiers) with a low inputwithstand voltage are used in the current-to-voltage converting circuitsof the respective voltage detecting apparatuses, it will still bepossible to avoid breakdown of the current-to-voltage convertingcircuits due to the potential difference between the detected ACvoltages and the reference voltages. This means that it is possible todetect line voltages while reducing the apparatus cost.

To achieve the second object, a second voltage detecting apparatus thatdetects a detected AC voltage generated in a detected object comprises:a detection electrode that is disposed facing the detected object and iscapacitively coupled to the detected object; a bootstrap circuit thatoperates using a floating power supply generated with a referencevoltage as a reference and outputs a detection signal whose amplitudechanges in accordance with an AC potential difference between thedetected AC voltage and the reference voltage; an insulating circuitthat inputs the detection signal and outputs an insulated detectionsignal that is electrically insulated from the detection signal; and avoltage generating circuit that amplifies the insulated detection signalso as to reduce the potential difference and thereby generates thereference signal.

In this second voltage detecting apparatus, in a state where thedetection electrode has been positioned facing the detected object, thebootstrap circuit outputs the detection signal that is based on, andwhose amplitude changes in accordance with, the potential differencebetween the AC voltage and the reference voltage. The insulating circuitoutputs the detection signal as the insulated detection signal that iselectrically insulated from the detection signal. The voltage generatingcircuit amplifies the insulated detection signal to generate thereference voltage. In this voltage detecting apparatus, since thecircuit connected to the detection electrode is the bootstrap circuitthat has an extremely high input impedance, the impedance of thedetection electrode is kept at a high impedance and the impedance of thefloating circuit unit as a whole that acts as a load of the voltagegenerating circuit also becomes a high impedance (that is, the load ofthe voltage generating circuit is light).

This means that according to the second voltage detecting apparatus, thevoltage generating circuit can cause the outputted reference voltage tofavorably follow a detected AC voltage with a short period (i.e., a highfrequency), and as a result, it is possible to precisely detect thedetected AC voltage across a wide frequency band.

In the second voltage detecting apparatus, the insulating circuit mayinclude an optical insulating element and/or a transformer, and outputthe detection signal, which is an analog signal, so as to beelectrically insulated from the input side thereof.

By using this construction, it becomes easy to electrically insulate(separate) the bootstrap circuit and the main circuit unit. Also, sincethe frequency characteristics are favorable over a wide range offrequencies when an optical insulating element is used, it is possibleto precisely detect the AC voltage of the detected object over a widerange of frequencies. Also, when a transformer is used as the insulatingcircuit, since a transformer normally has favorable frequencycharacteristics at a higher frequency band than an optical insulatingelement, it is possible to increase a maximum frequency of a frequencyband where the detected AC voltage can be detected. When the insulatingcircuit is constructed by connecting an optical insulating element and atransformer in parallel, by having the optical insulating elementoperate mainly at low frequencies and the transformer operate mainly athigh frequencies, it is possible to provide the insulating circuit withfavorable frequency characteristics over a significantly wider frequencyband. As a result, it is possible to detect the AC voltage of thedetected object with higher precision over a wide range of frequencies.

The second voltage detecting apparatus may further comprise a guardelectrode set at the reference voltage, and the bootstrap circuit and aprimary-side circuit of the insulating circuit are covered by the guardelectrode.

By using this construction, it is possible to make the circuits lesssusceptible to the effects of external magnetic fields, and as a result,it is possible to improve the detection precision of the detected ACvoltage.

In the second voltage detecting apparatus, an opening may be formed inthe guard electrode and the detection electrode may be disposed at aposition inside the guard electrode so as to face the opening in a statewhere the detection electrode does not protrude from the opening.

By disposing the detection electrode in a state where the detectionelectrode is inside the guard electrode at a position that faces anopening formed in the guard electrode without protruding from theopening (a “non-protruding state”), it is possible to make the detectionelectrode less susceptible to the effects of external magnetic fields.As a result, it is possible to significantly improve the detectionprecision of the detected AC voltage.

The second voltage detecting apparatus may further comprise an insulatorthat covers an entire surface of the detection electrode that faces thedetected object.

By using this construction, it is possible to reliably prevent shortingbetween the detected object and the detection electrode.

A second line voltage detecting apparatus comprises: a plurality ofsecond voltage detecting apparatuses that are constructed so that thedetection electrodes thereof are capable of being disposed facing aplurality of paths that correspond to the detected objects thereof andso as to be capable of detecting AC voltages generated on the respectivepaths as the detected AC voltages thereof; and a calculation unit thatcalculates a voltage difference between the AC voltages on two of thepaths detected by a pair of second voltage detecting apparatuses out ofthe plurality of second voltage detecting apparatuses and thereby findsa line voltage between the two paths.

According to this second line voltage detecting apparatus, by using thesecond voltage detecting apparatuses that include the bootstrapcircuits, it is possible to precisely detect the AC voltages of thepaths across a wide frequency band. This means it is also possible toprecisely measure the respective line voltages across a wide frequencyband.

To achieve the third object, a third voltage detecting apparatus thatdetects a detected AC voltage generated in a detected object comprises:a detection electrode that is disposed facing the detected object and iscapacitively coupled to the detected object; a detection unit thatoperates on a floating power supply generated with a voltage of areference voltage unit as a reference and outputs a detection signalwhose amplitude changes in accordance with an AC potential differencebetween the detected AC voltage and the voltage of the reference voltageunit; a standard signal outputting unit that outputs a standard signalto the reference voltage unit; an insulating unit that inputs thedetection signal and outputs an insulated detection signal that iselectrically insulated from the detection signal; a feedback controlunit that amplifies the insulated detection signal so as to reduce thepotential difference and thereby generates a feedback voltage andoutputs the feedback voltage to the reference voltage unit; and a signalextracting unit that amplifies the insulated detection signal by apredetermined gain to generate an amplified detection signal and, bycontrolling the gain so that a first signal component of the standardsignal included in the amplified detection signal and a second signalcomponent of the standard signal included in a reference signal based onthe voltage of the reference voltage unit become capable of cancelingone another out when the amplified detection signal and the referencesignal are subjected to one of addition and subtraction, also generatesa signal component of the detected AC voltage from the amplifieddetection signal and the reference signal and outputs the signalcomponent as an output signal.

According to the third voltage detecting apparatus, the standard signaloutputting unit outputs the standard signal to the reference voltageunit, and the detection unit that operates on a floating power supplyoutputs the detection signal whose amplitude changes in accordance withthe AC potential difference between the detected AC voltage and thevoltage of the reference voltage unit, and the insulating unit inputsthe detection signal and outputs the insulated detection signal. Basedon the insulated detection signal, the feedback control unit generatesthe feedback voltage so as to follow the detected AC voltage and outputsthe feedback voltage to the reference voltage unit. The signalextracting unit controls the amplitude of the insulated detection signalso that the amplitude of the first signal component of the standardsignal included in the generated amplified detection signal becomesequal to the amplitude of the second signal component of the standardsignal included in the reference signal, outputs the result as theamplified detection signal, and also adds the amplified detection signalwhose amplitude has been controlled in this way and the referencesignal. By doing so, the signal extracting unit generates a signalcomponent of the detected AC voltage from which the same signalcomponent as the standard signal has been removed and outputs thegenerated signal component as the output signal.

According to the third voltage detecting apparatus, since it is possibleto detect, based on the amplified detection signal generated by thesignal extracting unit, a detected AC voltage in a high frequency bandthat could not be detected by the detection operation carried out by thefeedback control unit alone, it becomes possible to detect the detectedAC voltage in a non-contact manner across a wider frequency band. Also,according to this voltage detecting apparatus, since it is possible todetect the output signal without being affected by the coupledcapacitance (capacitance) between the detected object and the detectionelectrode, it is possible to detect the AC voltage in a non-contactmanner without calculating the capacitance.

The third voltage detecting apparatus may further comprise a judgingunit that detects a level of a signal component of the standard signalincluded in one signal out of the insulated detection signal and theamplified detection signal and carries out at least one out of a judgingprocess that judges that the voltage detecting apparatus is operatingnormally when the detected level is equal to or above a set level and ajudging process that judges that the voltage detecting apparatus isoperating abnormally when the detected level is below the set level.

With this construction, since it is possible to carry out self-diagnosisas to whether the apparatus is operating normally based on the level ofthe signal component of the standard signal included in one signal outof the insulated detection signal and the amplified detection signal, itis possible to improve the reliability of the apparatus.

In the third voltage detecting apparatus, the signal extracting unit mayinclude: an amplifier circuit that amplifies the insulated detectionsignal by the gain to generate the amplified detection signal; asynchronous detection circuit that detects a wave detection signalshowing an amplitude of the signal component of the standard signalincluded in one of the amplified detection signal and the output signalby carrying out synchronous detection using the standard signal; and acontrol circuit that controls the gain of the amplifier circuit based onthe wave detection signal.

With this construction, since it is possible to accurately detect thesignal component of the standard signal by carrying out synchronousdetection, the signal component of the standard signal included in theamplified detection signal can be cancelled out with high precision. Bydoing so, it is possible to greatly reduce the signal component of thestandard signal included in the output signal. As a result, it ispossible to significantly improve the detection precision of thedetected AC voltage.

The third voltage detecting apparatus may further comprise a judgingunit that detects one level out of a level of a signal component of thestandard signal included in the output signal and a level of the wavedetection signal and carries out at least one out of a judging processthat judges that the voltage detecting apparatus is operating normallywhen the detected level is equal to or below a set level and a judgingprocess that judges that the voltage detecting apparatus is operatingabnormally when the detected level is above the set level.

With this construction, since it is possible to carry out self-diagnosisas to whether the apparatus is operating normally based on one level outof the level of the signal component of the standard signal included inthe output signal and the level of the wave detection signal, it ispossible to improve the reliability of the apparatus.

In the third voltage detecting apparatus, the signal extracting unit mayinclude one of an adder circuit that cancels out the first signalcomponent and the second signal component by carrying out the additionand outputs the output signal and a subtractor circuit that cancels outthe first signal component and the second signal component by carryingout the subtraction and outputs the output signal.

With this construction, it is possible to cancel out the first signalcomponent and the second signal component of the standard signal andoutput the output signal with a circuit that can be easily constructedusing an adder circuit or a subtractor circuit. Accordingly, it ispossible to reliably generate the output signal while simplifying theapparatus construction.

The third voltage detecting apparatus may further comprise a processingunit that detects the detected AC voltage based on the output signal.

With this construction, it is possible for example to have theprocessing unit detect the detected AC voltage at fixed intervals and/orhave a display apparatus or the like display the detected AC voltage asa waveform, thereby increasing convenience for the operator.

In the third voltage detecting apparatus, the processing unit maycalculate a voltage of the detected AC voltage based on the outputsignal.

With this construction, since it is possible for the processing unit tocalculate the voltage of the detected AC voltage based on the outputsignal, it is possible to accurately detect (measure) the detected ACvoltage.

The third voltage detecting apparatus may further comprise an amplitudeconverting unit that converts an amplitude of the reference signal andoutputs the reference signal with the converted amplitude to the signalextracting unit, and the signal extracting unit may control the gain sothat the first signal component and the second signal component includedin the reference signal with the converted amplitude are capable ofcanceling one another out.

With this construction, by multiplying the amplitude of the outputsignal that is composed of a signal component of the detected AC voltageby 1/k (where k is the ratio of the amplitude of the standard signalafter conversion to the amplitude of the standard signal beforeconversion), it is possible to detect the detected AC voltage.Accordingly by changing the ratio k, it is possible to increase thedetection range of the detected AC voltage.

A third line voltage detecting apparatus comprises: a plurality of thirdvoltage detecting apparatuses that are constructed so that the detectionelectrodes thereof are capable of being disposed facing a plurality ofpaths that correspond to the detected objects thereof and so as to becapable of detecting AC voltages generated on the respective paths asthe detected AC voltages thereof; and a calculation unit that calculatesa voltage difference between the AC voltages on two of the pathsdetected by a pair of third voltage detecting apparatuses out of theplurality of third voltage detecting apparatuses and thereby finds aline voltage between the two paths.

According to the third line voltage detecting apparatus, by using thevoltage detecting apparatus described above, even when the coupledcapacitances between the paths that are the detected objects and thedetection electrodes corresponding to such paths are unknown, it willstill be possible to accurately detect the line voltages between thepaths in a non-contact manner across a wider frequency band withoutcalculating the coupled capacitances (capacitances) between the pathsand the detection electrodes.

It should be noted that the disclosure of the present invention relatesto the contents of Japanese Patent Application 2008-158754 that wasfiled on 18 Jun. 2008, Japanese Patent Application 2009-105324 that wasfiled on 23 Apr. 2009, Japanese Patent Application 2009-105340 that wasfiled on 23 Apr. 2009, and Japanese Patent Application 2009-110302 thatwas filed on 30 Apr. 2009, the entire contents of which are hereinincorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will beexplained in more detail below with reference to the attached drawings,wherein:

FIG. 1 is a block diagram showing the construction of a first voltagedetecting apparatus;

FIG. 2 is a perspective view of the floating circuit unit appearing inFIGS. 1 and 10;

FIG. 3 is a schematic cross-sectional view taken along a line W-W inFIG. 2 for use in explaining the construction of the floating circuitunit;

FIG. 4 is a circuit diagram showing the construction of anothercurrent-to-voltage converting unit;

FIG. 5 is a circuit diagram showing the construction of anothercurrent-to-voltage converting unit;

FIG. 6 is a circuit diagram showing the construction of anothercurrent-to-voltage converting unit;

FIG. 7 is a block diagram showing the construction of another firstvoltage detecting apparatus;

FIG. 8 is a block diagram showing the construction of another firstvoltage detecting apparatus;

FIG. 9 is a block diagram showing the construction of first and secondline voltage detecting apparatuses that use the voltage detectingapparatuses shown in FIGS. 1 and 10;

FIG. 10 is a block diagram showing the construction of a second voltagedetecting apparatus;

FIG. 11 is a schematic cross-sectional view taken along a line W-W inFIG. 2 for use in explaining the construction of the floating circuitunit;

FIG. 12 is a circuit diagram showing another construction of a bootstrapcircuit;

FIG. 13 is a block diagram showing the construction of a third voltagedetecting apparatus;

FIG. 14 is a circuit diagram of a floating circuit unit appearing inFIG. 13;

FIG. 15 is a circuit diagram of an amplifier circuit appearing in FIG.13

FIG. 16 is a frequency characteristics graph of a voltage signal;

FIG. 17 is a frequency characteristics graph of a reference signal;

FIG. 18 is a frequency characteristics graph of an insulated detectionsignal outputted from a floating circuit unit of the third voltagedetecting apparatus;

FIG. 19 is a frequency characteristics graph of an amplified detectionsignal generated by the amplifier circuit;

FIG. 20 is a frequency characteristics graph of an output signal; and

FIG. 21 is a block diagram of a third line voltage detecting apparatusthat uses the third voltage detecting apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a voltage detecting apparatus and a linevoltage detecting apparatus will now be described with reference to theattached drawings.

First Embodiment

First, a voltage detecting apparatus (first voltage detecting apparatus)1 according to the present embodiment will now be described withreference to the drawings.

The voltage detecting apparatus 1 is a non-contact voltage detectingapparatus, includes a floating circuit unit 2 and a main circuit unit 3as shown in FIG. 1, and is constructed so as to be capable of detecting(measuring) an AC voltage V1 (detected AC voltage) generated in adetected object (measured object) 4 in a non-contact manner.

As shown in FIGS. 1 to 3, the floating circuit unit 2 includes a guardelectrode 21, a detection electrode 22, a current-to-voltage convertingunit CV, a driving circuit 25, and an insulating circuit 26. As oneexample in the present embodiment, the current-to-voltage convertingunit CV includes a current-to-voltage converting circuit 23 and anintegrating circuit 24. Also as one example in the present embodiment,the insulating circuit 26 is constructed of a photocoupler (and ishereinafter also referred to as the “photocoupler 26”). The guardelectrode 21 is constructed using a conductive material (as one example,a metal material), functions as a reference voltage unit in the floatingcircuit unit 2, and as one example encloses the circuits from thecircuit following the detection electrode 22 to the insulating circuit26, that is, the current-to-voltage converting circuit 23, theintegrating circuit 24, the driving circuit 25, and the photocoupler 26.By doing so, the circuits from the current-to-voltage converting circuit23 to the photocoupler 26 are covered by the guard electrode 21. Notethat the parts to be covered by the guard electrode 21 may be a rangefrom the circuit that follows the detection electrode 22 (i.e., acircuit connected to the detection electrode 22, in the presentembodiment the current-to-voltage converting circuit 23) to aprimary-side circuit of the photocoupler 26 (a light emitting diode,described later). This means that it is also possible to use aconstruction where a secondary-side circuit (a phototransistor,described later) of the photocoupler 26 is not covered by the guardelectrode 21. As one example, when an optical insulating element such asa photocoupler 26 constructed so that the primary-side circuit and thesecondary-side circuit are sealed in a single package using a resinmaterial is used, the photocoupler 26 is disposed with respect to theguard electrode 21 so that the part included in the primary-side circuitof the package (i.e., the light-emitting diode-side half of the package)is positioned inside the guard electrode 21 and the part included in thesecondary-side circuit (i.e., the phototransistor-side half of thepackage) projects outside (i.e., is exposed to) the periphery of theguard electrode 21. An opening (hole) 21 a is also formed in the guardelectrode 21, and in the present embodiment, as shown in FIGS. 2 and 3,the entire guard electrode 21 is covered by an insulating layer (oneexample of an insulator) 21 b. As one example, the detection electrode22 is formed in a plate-like shape and is disposed inside the guardelectrode 21 at a position facing the opening 21 a so as to not protrudeout of the guard electrode 21 from the opening 21 a (that is, anon-protruding state where the surface of the detection electrode 22 iswithdrawn from the surface of the guard electrode 21). Since aconstruction is used where the entire guard electrode 21 is covered bythe insulating layer 21 b and the detection electrode 22 is disposed ata position facing the opening 21 a, the insulating layer 21 b covers theentire surface of the detection electrode 22 that faces the detectedobject 4.

As one example, as shown in FIG. 1, the current-to-voltage convertingcircuit 23 includes a first operational amplifier 23 c with anon-inverted input terminal (“first input terminal”) connected via aresistor 23 a to the guard electrode 21 and an inverted input terminal(“second input terminal”) connected to the detection electrode 22, andalso includes a resistor 23 b connected between the inverted inputterminal and an output terminal of the first operational amplifier 23 cas a feedback circuit. The first operational amplifier 23 c of thecurrent-to-voltage converting circuit 23 is driven by being suppliedwith a positive voltage Vf+ and a negative voltage Vf−, described later.The first operational amplifier 23 c converts a detection current(hereinafter, “current signal”) I flowing between the detected object 4and the detection electrode 22 (more specifically, on a path between thedetection electrode 22 and the resistor 23 b) to a detection voltagesignal V2 and outputs the detection voltage signal V2. The currentsignal I has a current value that corresponds to a potential differenceVdi between an AC voltage V1 of the detected object 4 and a voltage(hereinafter “reference voltage”) of the guard electrode 21, or in otherwords, an AC potential difference, that is, a potential differencebetween an AC component of the AC voltage V1 and an AC component of thereference voltage. In this case, the amplitude of the detection voltagesignal V2 changes in proportion to the amplitude of the current signalI.

As one example, the integrating circuit 24 is constructed so as toinclude a second operational amplifier 24 d with a non-inverted inputterminal connected via the resistor 24 a to the guard electrode 21 andan inverted input terminal connected via an input resistor 24 b to anoutput terminal of the first operational amplifier 23 c, and alsoincludes a capacitor 24 c connected between the inverted input terminaland an output terminal of the second operational amplifier 24 d as afeedback circuit. In the integrating circuit 24, the second operationalamplifier 24 d operates by being supplied with the positive voltage Vf+and the negative voltage Vf− and integrates the detection voltage signalV2 to generate and output an integrated signal V3 whose voltage valuechanges in proportion to the potential difference Vdi between the ACvoltage V1 of the detected object 4 and the voltage (reference voltage)of the guard electrode 21. Note that in place of the constructiondescribed above, the integrating circuit 24 may be constructed of a lowpass filter.

Like the photocoupler 26, the driving circuit 25 is disposed so as tocome after the integrating circuit 24 and is disposed between theintegrating circuit 24 and a voltage generating circuit 34. As oneexample, the driving circuit 25 is constructed of a transistor (as oneexample in the present embodiment, an NPN bipolar transistor) 25 b witha base terminal connected to an output terminal of the secondoperational amplifier 24 d via an input resistor 25 a, a collectorterminal connected to the photocoupler 26, and an emitter terminalconnected to the negative voltage Vf−. The photocoupler 26 is includedin the concept of an “optical insulating element” that is one example ofan insulating circuit. A cathode terminal of a light-emitting diode thatis a primary-side circuit of the photocoupler 26 is connected to acollector terminal of the transistor 25 b and an anode terminal isconnected to the positive voltage Vf+. The phototransistor as thesecondary-side circuit of the photocoupler 26 is connected via wires W1to the main circuit unit 3. With this construction, by driving thephotocoupler 26 using the driving circuit 25 so as to operate in alinear region, the resistance value of the phototransistor of thephotocoupler 26 will change in accordance with (substantiallyproportionally to) the voltage value of the integrated signal V3.Accordingly, together with a resistor 33 of the main circuit unit 3,described later, the photocoupler 26 converts the integrated signal V3that is an analog signal inputted from the integrating circuit 24 to anintegrated signal V3 a that is a new analog signal and is electricallyinsulated from the integrated signal V3. Also, in place of thephotocoupler 26, it is also possible to use an optical MOS-FET that isalso an optical insulating element. In such case, the light-emittingdiode as the primary-side circuit of the optical MOS-FET is connected tothe transistor 25 b and the like in the same way as the light-emittingdiode of the photocoupler 26, and a FET pair as the secondary-sidecircuit is connected to the main circuit unit 3 via the wires W1.

As shown in FIG. 1, as one example, the main circuit unit 3 includes amain power supply circuit 31, a DC/DC converter (hereinafter simply“converter”) 32, a current-to-voltage converting resistor 33, thevoltage generating circuit 34, and a voltmeter 35. As one example, themain power supply circuit 31 is equipped with a battery and generates apositive voltage Vdd and a negative voltage Vss for driving thecomponent elements 32, 34 of the main circuit unit 3 (i.e., DC voltageswith respectively different polarities but with equal absolute valuesgenerated with ground G1 potential as a reference) from a DC voltage ofthe battery and outputs the voltages Vdd and Vss. As one example, theconverter 32 includes an insulated transformer that includes a primarycoil and a secondary coil that are electrically insulated from oneanother, a driving circuit that drives the primary coil of thetransformer, and a DC converting unit that rectifies and smooths an ACvoltage that is induced in the secondary coil of the transformer (noneof such component elements is shown), and is constructed as an insulatedpower supply where the secondary side is electrically insulated from theprimary side. In the converter 32, the driving circuit operates based onthe inputted positive voltage Vdd and the negative voltage Vss, and theprimary coil of the transformer to which the positive voltage Vdd isapplied is driven to induce an AC voltage in the secondary coil. The DCconverting unit rectifies and smooths such AC voltage. By doing so, thevoltages mentioned above (the positive voltage Vf+ and the negativevoltage Vf−) are generated in a floating state (a state where the groundG1, the positive voltage Vdd, and the negative voltage Vss areelectrically separated) with an internal reference potential (aninternal ground) as a reference. The positive voltage Vf+ and thenegative voltage Vf− generated in this way are supplied to the floatingcircuit unit 2 in a state where the internal ground mentioned above iselectrically connected to the guard electrode 21. Note that the positivevoltage Vf+ and the negative voltage Vf− are generated as DC voltageswith substantially equal absolute values but with different polarities.

One end of the current-to-voltage converting resistor 33 is connected tothe positive voltage Vdd and the other end of the current-to-voltageconverting resistor 33 is connected to the collector terminal of thephototransistor of the photocoupler 26. By doing so, the resistor 33 andthe phototransistor are connected in series between the positive voltageVdd and the negative voltage Vss. This means that when the resistancevalue of the phototransistor has changed in accordance with the voltagevalue of the integrated signal V3, by dividing the potential difference(Vdd−Vss) between the positive voltage Vdd and the negative voltage Vssbetween the resistance value of the resistor 33 and the resistance ofthe phototransistor, the integrated signal V3 a mentioned above isgenerated at the collector terminal of the phototransistor.

The voltage generating circuit 34 inputs and amplifies the integratedsignal V3 a to generate a voltage signal V4 (that is, a “referencevoltage”) and applies the voltage signal V4 to the guard electrode 21.Here, together with the guard electrode 21, the detection electrode 22,the current-to-voltage converting circuit 23, the integrating circuit24, the driving circuit 25, and the photocoupler 26 of the floatingcircuit unit 2, the voltage generating circuit 34 forms a feedback loopand generates the voltage signal V4 by carrying out an amplificationoperation that amplifies the integrated signal V3 a so as to cause thepotential difference Vdi to fall. In the present embodiment, as oneexample, the voltage generating circuit 34 includes an AC amplifiercircuit 34 a, a phase compensating circuit 34 b, and a booster circuit34 c. Here, the AC amplifier circuit 34 a inputs and amplifies theintegrated signal V3 a to generate a voltage signal V4 a. In this case,the AC amplifier circuit 34 a carries out an amplification operation togenerate the voltage signal V4 a, an absolute value of the voltage ofwhich changes in accordance with an increase or decrease in the absolutevalue of the voltage of the integrated signal V3 a. To stabilize thefeedback control operation (i.e., to prevent vibration), the phasecompensating circuit 34 b inputs the voltage signal V4 a, adjusts thephase thereof, and outputs the result as a voltage signal V4 b. As oneexample, the booster circuit 34 c is constructed using a boostingtransformer and boosts the voltage signal V4 b by a predetermined gain(i.e. by increasing the absolute voltage without changing the polarity)to generate the voltage signal V4 and applies the voltage signal V4 tothe guard electrode 21. The voltmeter 35 detects (measures) theeffective value of the voltage signal V4 and outputs the effective value(as one example, the voltmeter 35 displays the effective value on itsown display unit (not shown)).

Next, a detection operation (measurement operation) carried out on theAC voltage V1 of the detected object 4 by the voltage detectingapparatus 1 will now be described.

First, the floating circuit unit 2 (or the entire voltage detectingapparatus 1) is positioned near the detected object 4 so that thedetection electrode 22 faces but does not contact the detected object 4.By doing so, as shown in FIG. 1, the capacitance C0 is formed betweenthe detection electrode 22 and the detected object 4. Here, although thevalue of the capacitance C0 will change inversely proportionally to thedistance between the detection electrode 22 and the detected object 4,once the floating circuit unit 2 has been initially disposed, suchcapacitance C0 will be a constant (i.e., non-varying) value so long asthe environmental conditions, such as temperature, are constant. Sincethe value of the capacitance C0 is normally extremely small (forexample, a range of several pF to around several tens of pF or so), evenif the frequency of the AC voltage V1 is around several hundred Hz, theimpedance between the detected object 4 and the detection electrode 22will be sufficiently large (several MΩ). For this reason, in the voltagedetecting apparatus 1, even when the AC voltage V1 of the detectedobject 4 and the voltage of the guard electrode 21 (i.e., the voltagevalue of the voltage signal V4) greatly differ (i.e., when the potentialdifference Vdi is large), it will still be possible to use a low-costcomponent with a low input withstand voltage in the first operationalamplifier 23 c that constructs the current-to-voltage converting circuit23. Even when this construction is used, breakdown of the firstoperational amplifier 23 c due to the potential difference Vdi isavoided.

If, when the voltage detecting apparatus 1 is started, the potentialdifference Vdi between the AC voltage V1 of the detected object 4 andthe voltage of the guard electrode 21 (the reference voltage, here thevoltage of the voltage signal V4) has increased (for example, when thepotential difference Vdi has increased due to an increase in the ACvoltage V1), the magnitude of the current signal I that flows from thedetected object 4 via the detection electrode 22 into thecurrent-to-voltage converting circuit 23 also increases. In this case,the current-to-voltage converting circuit 23 reduces the voltage valueof the detection voltage signal V2 outputted by the current-to-voltageconverting circuit 23. In the integrating circuit 24, due to the fall inthe detection voltage signal V2, the current flowing from the outputterminal of the second operational amplifier 24 d via the capacitor 24 ctoward the inverted input terminal increases. For this reason, theintegrating circuit 24 increases the voltage of the integrated signalV3. Also, in keeping with the increase in the voltage of the integratedsignal V3, the transistor 25 b of the driving circuit 25 makes atransition to a deeper ON state. By doing so, in the photocoupler 26,the current flowing in the light-emitting diode increases and theresistance of the phototransistor falls. Accordingly, the voltage of theintegrated signal V3 a generated by dividing the potential difference(Vdd−Vss) between the resistance of the resistor 33 and the resistanceof the phototransistor also falls. Based on the integrated signal V3 a,the voltage generating circuit 34 increases the voltage of the voltagesignal V4 generated by the voltage generating circuit 34. In the voltagedetecting apparatus 1, the current-to-voltage converting circuit 23, theintegrating circuit 24, the driving circuit 25, the photocoupler 26, andthe main circuit unit 3 that construct a feedback loop in this waydetect the rise in the AC voltage V1 of the detected object 4 and carryout a feedback control operation that increases the voltage of thevoltage signal V4 so that the voltage of the guard electrode 21 (i.e.,the voltage of the voltage signal V4) follows the AC voltage V1.

Also, when the potential difference Vdi has increased due to a fall inthe AC voltage V1, the magnitude of the current signal I flowing outfrom the current-to-voltage converting circuit 23 via the detectionelectrode 22 to the detected object 4 increases. At this time, thecomponents such as the current-to-voltage converting circuit 23 thatconstruct the feedback loop carry out the opposite operation to thefeedback control operation described above to reduce the voltage of thevoltage signal V4 and thereby cause the voltage of the guard electrode21 (i.e., the voltage of the voltage signal V4) to follow the AC voltageV1. By doing so, in the voltage detecting apparatus 1, a feedbackcontrol operation that causes the voltage of the guard electrode 21(i.e., the voltage of the voltage signal V4) to follow the AC voltage V1is carried out in a short time, resulting in the voltage of the guardelectrode 21 (which due to virtual shorting of the first operationalamplifier 23 c, is also the voltage of the detection electrode 22) beingset equal to the AC voltage V1. The voltmeter 35 measures (detects) anddisplays the effective value of the voltage signal V4 (the “referencevoltage”, or voltage of the guard electrode 21) in real time.Accordingly, by observing the number displayed by the voltmeter 35, theoperator can detect (measure) the AC voltage V1 of the detected object4.

In this way, with the voltage detecting apparatus 1, in a state wherethe detection electrode 22 has been disposed facing the detected object4, the current signal I that flows between the detected object 4 and thecurrent-to-voltage converting circuit 23 (via the capacitance C0 formedbetween the detected object 4 and the detection electrode 22 and thedetection electrode 22 itself) with a current value in keeping with theAC potential difference between the AC voltage V1 and the voltage signalV4 (reference voltage) is converted by the current-to-voltage convertingcircuit 23 to the detection voltage signal V2. By integrating thedetection voltage signal V2 (more specifically, a current that flowsbased on the potential difference between the detection voltage signalV2 and the reference voltage) using the integrating circuit 24, theintegrated signal V3 whose amplitude changes in accordance with thepotential difference Vdi between the voltage of the detection electrode22 (i.e., the voltage of the guard electrode 21) and the AC voltage V1of the detected object 4 is generated. The integrated signal V3 is thenconverted using the photocoupler 26 to the integrated signal V3 a thatis electrically insulated from the integrated signal V3, the voltagesignal V4 is generated based on the integrated signal V3 a by thevoltage generating circuit 34, and the voltage signal V4 is applied tothe guard electrode 21.

According to the voltage detecting apparatus 1, since the magnitude ofthe capacitance C0 is normally extremely low (for example, in a range ofseveral pF to several tens of pF), it is possible to set the impedancebetween the detected object 4 and the detection electrode 22 at asufficiently high value (i.e., several MΩ). Since it is possible to setthe input impedance of the current-to-voltage converting circuit 23 at arelatively low value, it becomes less likely that an overvoltage will beapplied to the current-to-voltage converting circuit 23, so that even iflow-cost components with a low input withstand voltage are used in thecurrent-to-voltage converting circuit 23 of the current-to-voltageconverting unit CV (more specifically, even if a low-cost component witha low withstand voltage is used for the first operational amplifier 23 cthat constructs the current-to-voltage converting circuit 23), it willstill be possible to avoid breakdown of the first operational amplifier23 c due to the potential difference Vdi. That is, according to thevoltage detecting apparatus 1, it is possible to solve the first problemdescribed above and achieve the first object of the present invention.

Also, according to the voltage detecting apparatus 1, by using thephotocoupler 26 as the insulating circuit, it becomes easy toelectrically insulate (separate) the current-to-voltage convertingcircuit 23 and the following circuitry at an arbitrary position. Also,since the photocoupler 26 has favorable frequency characteristics over awide range of frequencies, it is possible to precisely detect (measure)the AC voltage V1 of the detected object 4 over a wide range offrequencies. Note that it is also possible to construct an insulatingcircuit using a transformer (such as a pulse transformer) in place of anoptical insulating element such as the photocoupler 26 and to constructan insulating circuit by connecting the photocoupler 26 and atransformer in parallel. With such constructions, the primary coil ofthe transformer functions as the primary-side circuit of the insulatingcircuit and the secondary coil functions as the secondary-side circuit.With the former construction, since a transformer will normally havefavorable frequency characteristics at a higher frequency band than thephotocoupler 26, by using a transformer when the frequency of the ACvoltage V1 is limited to high frequencies, it is possible to detect(measure) the AC voltage V1 precisely. With the latter construction, byhaving the photocoupler 26 operate mainly at low frequencies and thetransformer operate mainly at high frequencies, it is possible toprovide the insulating circuit with favorable frequency characteristicsover a wide frequency band. As a result, it is possible to detect(measure) the AC voltage V1 of the detected object 4 precisely over asignificantly wider range of frequencies.

Also, according to the voltage detecting apparatus 1, by using aconstruction where at least the circuits from the circuit that comesafter the detection electrode 22 to the primary-side circuit of theinsulating circuit (i.e., the photocoupler 26), or in other words, thecurrent-to-voltage converting circuit 23, the integrating circuit 24,the driving circuit 25, and the photocoupler 26 in the presentembodiment, are enclosed within the guard electrode 21 so as to becovered by the guard electrode 21, it is possible to protect suchcircuits from the effects of external magnetic fields. As a result, itis possible to improve the detection precision (i.e., measurementprecision) of the AC voltage V1.

According to the voltage detecting apparatus 1, by disposing thedetection electrode 22 inside the guard electrode 21 at a position thatfaces the opening 21 a formed in the guard electrode 21 but does notprotrude from the opening 21 a (a “non-protruding state” for the presentinvention), it is possible to protect the detection electrode 22 fromthe effects of external magnetic fields and thereby significantlyimprove the detection precision (i.e., measurement precision) of the ACvoltage V1.

Also, according to the voltage detecting apparatus 1, by using aconstruction where the entire surface of the detection electrode 22 thatfaces the detected object 4 is covered with the insulating layer 21 b asan insulator, it is possible to reliably prevent shorting between thedetected object 4 and the detection electrode 22.

Note that although a construction is used in the voltage detectingapparatus 1 described above where the detection electrode 22 is disposedinside the guard electrode 21 at a position that faces the opening 21 aformed in the guard electrode 21 so as to not protrude from the opening21 a (i.e., in a state where the detection electrode 22 is withdrawnfrom the surface of the guard electrode 21), if external magnetic fieldshave little effect, it is also possible to apply the present inventionto a construction where the detection electrode 22 is attached insidethe opening 21 a so as to be substantially flush with the guardelectrode 21 or so as to partially protrude from the guard electrode 21and to a construction where the detection electrode 22 is attached ontothe outer surface of the guard electrode 21 so as to cover the opening21 a. Note that with either construction, it should be obvious that thedetection electrode 22 is electrically insulated from the guardelectrode 21. Also, although a construction where the entire guardelectrode 21 is covered by the insulating layer 21 b has been describedabove, it is also possible to apply the present invention to aconstruction where only the detection electrode 22 that has a highprobability of contact with the detected object 4 is covered by theinsulating layer 21 b and other parts are not covered.

Also, in a construction where the main power supply circuit 31 issupplied with an AC voltage from outside and generates the positivevoltage Vdd and the negative voltage Vss, in place of a constructionwhere the converter 32 is supplied with the positive voltage Vdd and thenegative voltage Vss and generates the positive voltage Vf+ and thenegative voltage Vf− (i.e., a construction where DC is generated fromDC), it is possible to use a construction where the converter 32receives an AC voltage from outside in the same way as the main powersupply circuit 31 and generates the positive voltage Vf+ and thenegative voltage Vf−. Note that in such construction where DC isgenerated from AC, by using a transformer in the same way as the exampledescribed above, the converter 32 is constructed as an insulated powersupply where the primary side and the secondary side are electricallyinsulated from one another.

Also, although a construction where the photocoupler 26 and the drivingcircuit 25 thereof are disposed so as to come after the integratingcircuit 24 and the integrated signal V3 is converted to the integratedsignal V3 a that is electrically insulated from the integrated signal V3has been described above as an example, it is also possible to apply thepresent invention to a construction where the photocoupler 26 and thedriving circuit 25 thereof are disposed so as to come before theintegrating circuit 24, that is, between the current-to-voltageconverting circuit 23 and the integrating circuit 24, and the detectionvoltage signal V2 is converted to a new detection voltage signal that iselectrically insulated from the detection voltage signal V2 andoutputted to the integrating circuit 24. It is also possible to applythe present invention to a construction where other circuits aside fromthe driving circuit 25, the photocoupler 26, and the integrating circuit24 are added as appropriate between the main circuit unit 3 and thevoltage generating circuit 34. When such construction is used, theinsulating circuit (i.e., an optical insulating element and/or atransformer) is interposed on a path so as to receive a signal generatedby any of the circuits present between the current-to-voltage convertingcircuit 23 and the voltage generating circuit 34 and output a signalthat is electrically insulated from the inputted signal. Also, asdescribed earlier, the part to be covered by the guard electrode 21 isthe range from the circuit that comes after the detection electrode 22(in the present embodiment, the current-to-voltage converting circuit23) to the primary-side circuit of the photocoupler 26, that is, theprimary-side circuit of the insulating circuit. This means that when theposition of the insulating circuit has changed as described above, thecircuits to be covered by the guard electrode 21 will also change inaccordance with the disposed position of the insulating circuit. Forexample, in a construction where the photocoupler 26 that is theinsulating circuit and the driving circuit 25 thereof are disposed so asto come before the integrating circuit 24, the current-to-voltageconverting circuit 23, the driving circuit 25 and the primary-side ofthe photocoupler 26 will be covered by the guard electrode 21. Asanother example, it is also possible to apply the present invention to aconstruction equipped, together with a construction where the voltmeter35 detects (measures) and outputs the effective value of the voltagesignal V4, or in place of such construction, with a DSP (Digital SignalProcessor) that samples the voltage signal V4 and displays the waveformof the voltage signal V4 on a display unit.

Also, in place of the construction described above, thecurrent-to-voltage converting unit CV may be realized using theconstruction shown in FIG. 4 or in FIG. 5. More specifically, in thecurrent-to-voltage converting unit CV shown in FIG. 4, thecurrent-to-voltage converting circuit 23 is composed of the firstoperational amplifier 23 c constructed of a voltage follower and aresistor 23 d disposed between the non-inverted input terminal of thefirst operational amplifier 23 c and the guard electrode 21 (thereference voltage unit). By doing so, it is possible to convert thecurrent signal I to a voltage V5 using the resistor 23 d, with the firstoperational amplifier 23 c (amplifier) that functions as a buffer (i.e.,an amplifier with a gain of 1) outputting such voltage V5 (that is, thevoltage generated across both ends of the resistor 23 d) as thedetection voltage signal V2. Also, in the current-to-voltage convertingunit CV shown in FIG. 5, the current-to-voltage converting circuit 23 isconstructed of the first operational amplifier 23 c, the resistor 23 ddisposed between the non-inverted input terminal of the firstoperational amplifier 23 c and the guard electrode 21 (reference voltageunit), a resistor 23 e disposed between the inverted input terminal ofthe first operational amplifier 23 c and the guard electrode 21, and theresistor 23 b disposed as a feedback circuit between the output terminaland the inverted input terminal of the first operational amplifier 23 c.By doing so, the current signal I can be converted to the voltage V5 bythe resistor 23 d and the voltage V5 (i.e., the voltage generated acrossboth ends of the resistor 23 d) is outputted as the detection voltagesignal V2 by the first operational amplifier 23 c (amplifier) thatfunctions as a non-inverting amplifier. Accordingly, with voltagedetecting apparatuses 1 that use the current-to-voltage converting unitsCV shown in FIGS. 4 and 5 also, it is possible to achieve the sameoperational effects as the voltage detecting apparatus 1 equipped withthe current-to-voltage converting unit CV of the construction shown inFIG. 1.

Also, although an example where the current-to-voltage converting unitCV is constructed of the current-to-voltage converting circuit 23 andthe integrating circuit 24 has been described above, as shown in FIG. 6,it is also possible to construct the current-to-voltage converting unitCV of a single integrating circuit 27. The integrating circuit 27includes a function as a current-to-voltage converting circuit and afunction as an integrating circuit, and as one example, has theconstruction of the current-to-voltage converting circuit 23 shown inFIG. 1 as a base construction, with a capacitor 27 a also beingconnected in parallel to the resistor 23 b. In this case, the capacitor27 a is constructed of a capacitor with a capacitance of around 0.01 μF,for example, and the resistor 23 b is constructed of a resistor with ahigh resistance of around 1 MΩ, for example. This means that in theintegrating circuit 27, due to the current signal I mainly flowing inthe capacitor 27 a, an integrating operation is carried out at the sametime as the current-to-voltage converting operation, and the integratedsignal V3 whose voltage changes in proportion to the potentialdifference Vdi between the AC voltage V1 of the detected object 4 andthe voltage (reference voltage) of the guard electrode 21 is generated.Accordingly, with a voltage detecting apparatus 1 that uses thecurrent-to-voltage converting unit CV with the construction shown inFIG. 6 also, it is possible to achieve the same operational effects asthe voltage detecting apparatus 1 that uses the current-to-voltageconverting unit CV of the construction shown in FIG. 1. Note that in theintegrating circuit 27 described above, the resistor 23 b functions soas to suppress the drop in dynamic range since there is the risk that ifonly the capacitor 27 a were used, the amount of feedback would becomeremarkably low in a state close to DC and the gain would becomeextremely high, resulting in saturation of the first operationalamplifier 23 c by the offset produced by the bias current. Also,although the non-inverted input terminal of the first operationalamplifier 23 c and the guard electrode 21 are connected via the resistor23 a, it is also possible to use a construction where the non-invertedinput terminal and the guard electrode 21 are directly connected.

Although the voltage detecting apparatus 1 that uses a constructionwhere the integrated signal V3 that is an analog signal generated by thefloating circuit unit 2 is outputted to the main circuit unit 3 as theintegrated signal V3 a that is an analog signal that is electricallyinsulated from the integrated signal V3 has been described above, it isalso possible to apply the present invention to a construction where theintegrated signal V3 is converted to a digital signal and outputted tothe main circuit unit 3. Other first voltage detecting apparatuses 1A,1B that use such construction will now be described with reference toFIGS. 7 and 8. Note that components that are the same as in the voltagedetecting apparatus 1 described above have been assigned the samereference numerals and description thereof is omitted.

First, the voltage detecting apparatus (another first voltage detectingapparatus) 1A will be described. As shown in FIG. 7, the voltagedetecting apparatus 1A includes a floating circuit unit 2A and a maincircuit unit 3A, and is constructed so as to be capable of detecting(measuring) the AC voltage V1 generated in the detected object 4 in anon-contact manner. As shown in FIG. 7, the floating circuit unit 2Aincludes the guard electrode 21, the detection electrode 22, thecurrent-to-voltage converting unit CV, a buffer amplifier 28, an A/Dconverting circuit 29, and an insulating circuit 26A. Thecurrent-to-voltage converting unit CV may be any of the circuits shownin FIGS. 1, 4, 5, and 6, and as one example in the present embodiment,is constructed using the integrating circuit 27 shown in FIG. 6. Thebuffer amplifier 28 is constructed of an amplifier with a high inputimpedance and a low output impedance, receives an input of theintegrated signal V3 outputted from the integrating circuit 27, andoutputs the integrated signal V3 with a low impedance. The A/Dconverting circuit 29 is constructed of an A/D converter and samples theintegrated signal V3 with a predetermined sampling period (a period thatis sufficiently shorter than the period of the AC voltage V1) to convertthe integrated signal V3 to digital data D1 that shows the voltagewaveform of the integrated signal V3 and outputs the digital data D1.The insulating circuit 26A is constructed using a digital isolator (andis hereinafter referred to as the “digital isolator 26A”). The digitalisolator 26A is a device that transmits the digital signal between aninput and an output side in an electrically insulated manner. Examplesof digital isolators include devices constructed using opticallyinsulated isolators such as phototransistors and photocouplers anddevices constructed using magnetically coupled isolators. In the presentembodiment, the digital isolator 26A converts the digital data D1outputted from the A/D converting circuit 29 to digital data D1 a thatis electrically insulated from the digital data D1 and outputs thedigital data D1 a via the wire W1 to the main circuit unit 3A.

As shown in FIG. 7, as one example, the main circuit unit 3A includesthe main power supply circuit 31, the DC/DC converter 32, a voltagegenerating circuit 34A, and the voltmeter 35. The voltage generatingcircuit 34A includes a processing unit 34 d, a D/A converting circuit 34e, and the booster circuit 34 c. The processing unit 34 d is constructedof a CPU or DSP (Digital Signal Processor), and carries out digitalprocessing on the inputted digital data D1 a, such as the amplificationprocess and the phase adjusting process that are carried out by the ACamplifier circuit 34 a and the phase compensating circuit 34 b of thevoltage detecting apparatus 1 described earlier, and outputs theprocessed digital data D1 a as new digital data D2. The D/A convertingcircuit 34 e receives input of the digital data D2 and converts thedigital data D2 to an analog signal to generate and output the samesignal as the voltage signal V4 b generated by the phase compensatingcircuit 34 b in the voltage detecting apparatus 1. The booster circuit34 c boosts the voltage signal V4 b by a predetermined gain to generatethe voltage signal V4 and applies the voltage signal V4 to the guardelectrode 21.

According to the voltage detecting apparatus 1A also, in the same way asthe voltage detecting apparatus 1, it is possible to detect (measure)the AC voltage V1 of the detected object 4 via the capacitance C0 thatis normally extremely small. This means that even if a low-costcomponent with a low input withstand voltage is used for the firstoperational amplifier 23 c that constructs the current-to-voltageconverting circuit 23 of the current-to-voltage converting unit CV, itwill still be possible to avoid breakdown of the first operationalamplifier 23 c due to the potential difference Vdi. Also, in the voltagedetecting apparatus 1A, since the digital isolator 26A is used as aninsulating circuit to convert the integrated signal V3 outputted fromthe integrating circuit 27 to the digital data D1 and the digital dataD1 is outputted to the main circuit unit 3A as the digital data D1 a soas to be electrically insulated from the digital data D1, compared to aconstruction that outputs the integrated signal V3 as an analog signal,it is possible to transmit information on the integrated signal V3(i.e., the voltage waveform) with high precision to the main circuitunit 3A without being affected by the temperature, changes over time,and the like for the transfer path (the wire W1). As a result, it ispossible to improve the detection precision (measurement precision) ofthe AC voltage V1.

Note that although not shown, it is also possible to use a constructionthat generates the voltage signal V4 using a voltage generating circuitconstructed of the D/A converting circuit 34 e and the componentelements of the voltage generating circuit 34 of the voltage detectingapparatus 1 (i.e., the AC amplifier circuit 34 a, the phase compensatingcircuit 34 b, and the booster circuit 34 c), where the D/A convertingcircuit 34 e receives a direct input of the digital data D1 a andconverts the digital data D1 a to an analog signal, and the AC amplifiercircuit 34 a, the phase compensating circuit 34 b, and the boostercircuit 34 c operate in the same way as in the voltage detectingapparatus 1 on such analog signal.

Next, a voltage detecting apparatus (another first voltage detectingapparatus) 1B will be described. Note that components that are the sameas in the voltage detecting apparatuses 1, 1A described above have beenassigned the same reference numerals and description thereof is omitted.

As shown in FIG. 8, the voltage detecting apparatus 1B includes afloating circuit unit 2B and a main circuit unit 3B, and is constructedso as to be capable of detecting (measuring) the AC voltage V1 generatedin the detected object 4 in a non-contact manner. As shown in FIG. 8,the floating circuit unit 2B includes the guard electrode 21, thedetection electrode 22, the current-to-voltage converting unit CV, thebuffer amplifier 28, a PWM signal generating circuit 29A, and theinsulating circuit 26A (the digital isolator 26A). In the same way asthe voltage detecting apparatus 1A, the current-to-voltage convertingunit CV is constructed for example using the integrating circuit 27shown in FIG. 6. The buffer amplifier 28 receives an input of theintegrated signal V3 and outputs the integrated signal V3 with lowimpedance. The PWM signal generating circuit 29A subjects the inputtedintegrated signal V3 to pulse width modulation to generate and output apulse signal (a binary signal (digital signal), or in other words, asignal where a “High” level and a “Low” level are set at predeterminedvoltage values) Sp whose pulse width changes in accordance with thevoltage value of the integrated signal V3. As one example in the presentembodiment, the PWM signal generating circuit 29A includes a triangularwave generating circuit that generates a triangular wave with a fixedperiod and a comparator that compares the triangular wave and theinputted integrated signal V3 and generates the pulse signal Sp (neitherthe triangular wave generating circuit nor the comparator is shown). Asone example, in the present embodiment, the PWM signal generatingcircuit 29A generates the pulse signal Sp with the same period as thetriangular wave so that the pulse width decreases when the voltage valueof the integrated signal V3 has increased and the pulse width increaseswhen the voltage value of the integrated signal V3 has decreased. Thedigital isolator 26A converts the pulse signal Sp to a pulse signal Spathat is electrically insulated from the pulse signal Sp and outputs thepulse signal Spa to the main circuit unit 3B via the wire W1.

As shown in FIG. 8, as one example the main circuit unit 3B includes themain power supply circuit 31, the DC/DC converter 32, a voltagegenerating circuit 34B, and the voltmeter 35. As one example in thepresent embodiment, as shown in FIG. 8, the voltage generating circuit34B is constructed as a boosting circuit and includes a driving circuit34 f that includes a switching element that is controlled by the pulsesignal Spa so as to switch on and off, a boosting transformer 34 g thathas a primary coil and a secondary coil and whose primary coil is drivenby the driving circuit 34 f, and a DC converting circuit 34 h thatgenerates the voltage signal V4 by rectifying and smoothing the ACvoltage induced in the secondary coil of the boosting transformer 34 g.By using this construction, the voltage generating circuit 34B iscontrolled so as to shorten the On period of the switching element whenthe pulse width of the pulse signal Spa has decreased to lower thevoltage signal V4, and so as to extend the On period of the switchingelement when the pulse width of the pulse signal Spa has increased toraise the voltage signal V4. The voltage generating circuit 34B appliesthe voltage signal V4 to the guard electrode 21.

This means that according to the voltage detecting apparatus 1B also, inthe same way as the voltage detecting apparatuses 1, 1A, it is possibleto detect (measure) the AC voltage V1 of the detected object 4 via thecapacitance C0 that is normally extremely small. As a result, it ispossible to avoid breakdown of the first operational amplifier 23 c dueto the potential difference Vdi, even if a low-cost component with a lowwithstand voltage is used for the first operational amplifier 23 c thatconstructs the integrating circuit 27 of the current-to-voltageconverting unit CV. Also, in the voltage detecting apparatus 1B, in thesame way as in the voltage detecting apparatus 1A, by using the digitalisolator 26A as the insulating circuit, the integrated signal V3outputted from the integrating circuit 27 is converted to the pulsesignal Sp that is a binary signal and the pulse signal Sp is outputtedto the main circuit unit 3B as the pulse signal Spa that is electricallyinsulated from the pulse signal Sp. This means that compared to aconstruction where the integrated signal V3 is outputted as an analogsignal, it is possible to transmit information (voltage waveform) on theintegrated signal V3 to the main circuit unit 3B with high precisionwithout being affected by the temperature, changes over time, and thelike of the transfer route (the wire W1), and as a result it is possibleto improve the detection precision (measurement precision) of the ACvoltage V1.

Next, a line voltage detecting apparatus (first line voltage detectingapparatus) 51 equipped with a plurality of the voltage detectingapparatuses 1, the voltage detecting apparatuses 1A, or the voltagedetecting apparatuses 1B described above will be described. Note thatthe line voltage detecting unit 51 may be constructed of only thevoltage detecting apparatuses 1, only the voltage detecting apparatuses1A, or only the voltage detecting apparatuses 1B, of two types ofapparatus selected from the voltage detecting apparatus 1, the voltagedetecting apparatus 1A, and the voltage detecting apparatus 1B, or amixture of all such types of apparatus. In the following descriptionhowever, an example where the line voltage detecting apparatus 51 isconstructed of only the voltage detecting apparatuses 1 will bedescribed.

First, the line voltage detecting apparatus 51 will be described withreference to the drawings. Note that an example where line voltages ofthree-phase (R phase, S phase, and T phase), three-wire AC paths(hereinafter simply “paths”) R, S, T are detected (measured) will bedescribed.

As one example, as shown in FIG. 9, the line voltage detecting unit 51includes an equal number of (here, three) voltage detecting apparatuses1 to the number of paths R, S, T (hereinafter, the voltage detectingapparatuses 1 are referred to individually as the voltage detectingapparatuses 1 r, 1 s, 1 t corresponding to the paths R, S, T and as the“voltage detecting apparatuses 1” when no special distinction isrequired), a calculation unit 52, and a display unit 53, and isconstructed so as to be capable of detecting (measuring) a line voltageVrs between the paths R, S, a line voltage Vst between the paths S, T,and a line voltage Vrt between the paths R, T in a non-contact manner.

As shown in FIG. 9, the respective voltage detecting apparatuses 1 havethe same construction and are each equipped with the floating circuitunit 2 and the main circuit unit 3 described above, have the paths R, S,T as the respective detected objects thereof, and carry out feedbackcontrol to match the respective voltage signals V4 to the AC voltagesVrp, Vsp, Vtp (detected AC voltages) on such paths. In the voltagedetecting apparatuses 1 r, 1 s, it in the present embodiment, therespective voltmeters 35 output voltage data Dva, Dvb, Dvc showing thewaveforms of the voltage signals V4 that have been detected (measured).Hereinafter, when no special distinction is required, the voltage dataDva, Dvb, Dvc are collectively referred to as the “voltage data Dv”.

The calculation unit 52 is constructed of a calculation circuit thatincludes a CPU and a memory (neither of which is shown), and carries outa line voltage calculating process that finds (calculates) the linevoltages based on the voltage data Dv outputted from the respectivevoltage detecting apparatuses 1. The calculation unit 52 displays theresult of the line voltage calculating process on the display unit 53.In the present embodiment, the display unit 53 is constructed of amonitor apparatus such as a liquid crystal display. Note that it is alsopossible to construct the display unit 53 of a print apparatus such as aprinter. Also, the ground G1 of each main circuit unit 3 is connected toone another, as described later. The calculation unit 52 and the displayunit 53 operate by being supplied with the positive voltage Vdd and thenegative voltage Vss from the main power supply circuit 31 included inthe main circuit unit 3 of one out of the three main circuit units 3. Byusing this construction, the floating circuit units 2, the main circuitunits 3, the calculation unit 52, and the display unit 53 are placed inan electrically floating state from the Earth.

Next, a detection operation (measurement operation) by the line voltagedetecting apparatus 51 will be described.

First, during detection (measurement), as shown in FIG. 9, to detect(measure) the AC voltage Vrp of the path R using the voltage detectingapparatus 1 r, the floating circuit unit 2 of the voltage detectingapparatus 1 r is positioned near the path R and the detection electrode22 of the voltage detecting apparatus 1 r is positioned opposite thecorresponding path R. In the same way, to detect (measure) the ACvoltages Vsp, Vtp of the paths S, T, the detection electrodes 22 of therespective floating circuit units 2 of the other voltage detectingapparatuses is, it are positioned opposite the corresponding paths S, T.By doing so, the capacitance C0 (see FIG. 1) is formed between each ofthe respective detection electrodes 22 and the paths R, S, T. Here,although the respective values of the capacitance C0 will changeinversely proportionately to the distances between the respectivedetection electrodes 22 and the cores of the paths R, S, T, after thefloating circuit units 2 have been disposed, such capacitances C0 willbe constant (i.e., not vary) so long as environmental conditions such ashumidity are constant. Also, since the respective capacitances C0 arenormally extremely low (for example, in a range of several pF to severaltens of pF), the impedances between the paths R, S, T and the respectivedetection electrodes 22 will be sufficiently large (several MΩ). Bydoing so, in the line voltage detecting apparatus 51 also, even if thepotential differences Vdi between the paths R, S, T and thecorresponding detection electrodes 22 are large, breakdown of the firstoperational amplifier 23 c in any of the voltage detecting apparatuses 1due to the AC voltages Vrp, Vsp, Vtp of the paths R, S, T is prevented.Also, by connecting (shorting) the ground G1 of each voltage detectingapparatuses 1, the potential of the ground G1 in each voltage detectingapparatus 1 is set equal.

Next, when the line voltage detecting apparatus 51 is started, in thevoltage detecting apparatus 1 r, the current-to-voltage convertingcircuit 23, the integrating circuit 24, the driving circuit 25, thephotocoupler 26, and the main circuit unit 3 that construct the feedbackloop carry out a feedback control operation that changes the voltage ofthe voltage signal V4 in keeping with the changes in the AC voltage Vrpof the path R. By doing so, the voltage of the guard electrode 21 (whichis also the voltage of the voltage signal V4 and the voltage of thedetection electrode 22) will follow the AC voltage Vrp. In the voltagedetecting apparatuses 1 s, 1 t also, the current-to-voltage convertingcircuit 23, the integrating circuit 24, the driving circuit 25, thephotocoupler 26, and the main circuit unit 3 that construct the feedbackloop carry out a feedback control operation that changes the voltage ofthe voltage signal V4 in keeping with the changes in the AC voltagesVsp, Vtp of the paths S, T. By doing so, the voltages of the respectiveguard electrodes 21 (which are also the voltages of the respectivevoltage signals V4 and the voltages of the respective detectionelectrodes 22) will follow the AC voltages Vsp, Vtp. Also, thevoltmeters 35 of the respective voltage detecting apparatuses 1continuously output the voltage data Dva, Dvb, Dvc that respectivelyshow the waveforms of the voltages of the voltage signals V4 that havebeen detected (measured), that is, the AC voltages Vrp, Vsp, Vtp of thepaths R, S, T.

The calculation unit 52 inputs the voltage data Dva, Dvb, Dvc outputtedfrom the respective voltage detecting apparatuses 1 and stores the datain the memory. Next, the calculation unit 52 carries out a line voltagecalculating process. More specifically, the calculation unit 52calculates the voltage difference between the voltage data Dva, Dvb tofind (calculate) the line voltage Vrs between the paths R, S. In thesame way, the calculation unit 52 calculates the voltage differencebetween the voltage data Dvb, Dvc to find (calculate) the line voltageVst between the paths S, T and calculates the voltage difference betweenthe voltage data Dva, Dvc to find (detect) the line voltage Vrt betweenthe paths R, T. In this case, as described earlier, since the ACvoltages Vrp, Vsp, Vtp of the paths R, S, T are detected (measured) bythe respective voltage detecting apparatuses 1 with the shared ground G1as a reference, by calculating the voltage differences between the ACvoltages Vrp, Vsp, Vtp, the line voltages Vrs, Vst, Vrt can beaccurately found (calculated) regardless of the potential of thereference potential. The calculation unit 52 also has the calculatedline voltages Vrs, Vst, Vrt displayed on the display unit 53.

In this case according to the line voltage detecting apparatus 51, byusing the voltage detecting apparatuses 1, even if a low-cost componentwith a low input withstand voltage is used for the first operationalamplifier 23 c that constructs the current-to-voltage converting circuit23 in each voltage detecting apparatus 1, it will still be possible toavoid breakdown of the first operational amplifier 23 c due to thepotential difference Vdi. As a result, it is possible to detect(measure) the line voltages Vrs, Vst, Vrt while reducing the apparatuscost.

Note that the present invention is not limited to the aboveconstruction. For example, although an example has been described wherea plurality of the voltage detecting apparatuses 1 of the sameconstruction that are respectively equipped with the main power supplycircuit 31 and the converter 32 are used, it is also possible to applythe present invention to a construction where the main power supplycircuit 31 and the converter 32 are provided in one out of the pluralityof voltage detecting apparatuses 1 and the positive voltage Vdd, thenegative voltage Vss, the positive voltage Vf+, and the negative voltageVf− are supplied from such voltage detecting apparatus 1 to the othervoltage detecting apparatuses 1.

Second Embodiment

A second embodiment of a voltage detecting apparatus and a line voltagedetecting apparatus will now be described with reference to the attacheddrawings.

First, a voltage detecting apparatus (second voltage detectingapparatus) 101 according to the present embodiment will be describedwith reference to the drawings. Note that components that are the sameas in the voltage detecting apparatus 1 described above have beenassigned the same reference numerals and description thereof is omitted.

The voltage detecting apparatus 101 is a non-contact voltage detectingapparatus, includes a floating circuit unit 2C and a main circuit unit 3as shown in FIG. 10, and is constructed so as to be capable of detecting(measuring) an AC voltage V1 (detected AC voltage) generated in adetected object (measured object) 4 in a non-contact manner.

As shown in FIGS. 10, 11, a floating circuit unit 2C includes the guardelectrode 21, the detection electrode 22, a bootstrap circuit 27, thedriving circuit 25, and the insulating circuit 26. As one example in thepresent embodiment, the insulating circuit 26 is constructed of aphotocoupler (and is hereinafter also referred to as the “photocoupler26”). The guard electrode 21 is constructed using a conductive material(for example, a metal material) as a reference voltage unit in thefloating circuit unit 2C, and as one example, the bootstrap circuit 27,the driving circuit 25, and the photocoupler 26 are enclosed inside theguard electrode 21. By doing so, the circuitry from the bootstrapcircuit 27 to the photocoupler 26 is covered by the guard electrode 21.Note that the part to be covered by the guard electrode 21 may be thecircuitry from the bootstrap circuit 27 to the primary-side circuit ofthe photocoupler 26 (the light-emitting diode described later). Thismeans that it is also possible to use a construction where thesecondary-side circuit of the photocoupler 26 (the phototransistordescribed later) is not covered by the guard electrode 21. Note thatsince the specific construction of the guard electrode 21 is the same asin the voltage detecting apparatus 1, description thereof is omitted.

As one example, as shown in FIG. 10, the bootstrap circuit 27 includesan operational amplifier 27 a, two resistors 27 b, 27 c that aredisposed so as to be connected in series between the non-inverted inputterminal of the operational amplifier 27 a and the guard electrode 21, acapacitor 27 d that is disposed between the inverted input terminal ofthe operational amplifier 27 a and the connection point between theresistors 27 b, 27 c, and a resistor 27 e disposed between the invertedinput terminal and the output terminal of the operational amplifier 27a. The detection electrode 22 is connected to the non-inverted inputterminal of the operational amplifier 27 a. In the bootstrap circuit 27,the operational amplifier 27 a operates by being supplied with thepositive voltage Vf+ and the negative voltage Vf− described later,inputs the voltage generated in the detection electrode 22 and outputs adetection voltage signal (detection signal) V2 with a voltage inaccordance with the potential difference Vdi between the AC voltage V1of the detected object 4 and the voltage (reference voltage) of theguard electrode 21. In this case, the amplitude of the detection voltagesignal V2 changes in proportion to the amplitude of the potentialdifference Vdi.

The driving circuit 25 is disposed together with the photocoupler 26 soas to come after the bootstrap circuit 27. As one example, the drivingcircuit 25 is constructed of a transistor (as one example in the presentembodiment, an NPN bipolar transistor) 25 b with a base terminalconnected to the output terminal of the operational amplifier 27 a viathe input resistor 25 a, a collector terminal connected to thephotocoupler 26, and an emitter terminal connected to the negativevoltage Vf−. By using this construction, by driving the photocoupler 26using the driving circuit 25 so as to operate in a linear region, theresistance value of the phototransistor in the photocoupler 26 willchange in accordance with (substantially proportionally) to the voltageof the detection voltage signal V2. Accordingly, together with theresistor 33 of the main circuit unit 3 described later, the photocoupler26 converts the detection voltage signal V2 inputted from the bootstrapcircuit 27 to a new detection voltage signal (an insulated detectionsignal) V2 a that is electrically insulated from the detection voltagesignal V2.

As shown in FIG. 10, as one example, the main circuit unit 3 includesthe main power supply circuit 31, the converter 32, the resistor 33 forconverting a current to a voltage, the voltage generating circuit 34,and the voltmeter 35, and has the same construction as the main circuitunit 3 of the voltage detecting apparatus 1.

One end of the current-to-voltage converting resistor 33 is connected tothe positive voltage Vdd and the other end of the current-to-voltageconverting resistor 33 is connected to the collector terminal of thephototransistor of the photocoupler 26. By doing so, the resistor 33 andthe phototransistor are connected in series between the positive voltageVdd and the negative voltage Vss. This means that when the resistancevalue of the phototransistor has changed in accordance with the voltagevalue of the detection signal V2, by dividing the potential difference(Vdd−Vss) between the positive voltage Vdd and the negative voltage Vssbetween the resistance value of the resistor 33 and the resistance ofthe phototransistor, the detection signal V2 a mentioned above isgenerated at the collector terminal of the phototransistor.

By inputting and amplifying the detection voltage signal V2 a, thevoltage generating circuit 34 generates the voltage signal V4 (that is,the reference voltage) and applies the voltage signal V4 to the guardelectrode 21. In this case, together with the guard electrode 21, thedetection electrode 22, the bootstrap circuit 27, the driving circuit25, and the photocoupler 26 of the floating circuit unit 2C, the voltagegenerating circuit 34 forms a feedback loop, and by carrying out anamplification operation that amplifies the detection voltage signal V2 aso as to reduce the potential difference Vdi, the voltage generatingcircuit 34 generates the voltage signal V4. In the present embodiment,as one example, the voltage generating circuit 34 includes the ACamplifier circuit 34 a, the phase compensating circuit 34 b, and thebooster circuit 34 c. Here, the AC amplifier circuit 34 a inputs andamplifies the detection voltage signal V2 a to generate the voltagesignal V4 a. In this case, the AC amplifier circuit 34 a carries out anamplification operation on the voltage signal V4 a whose absolutevoltage value changes in accordance with increases and decreases in theabsolute voltage of the detection voltage signal V2 a to generate thevoltage signal V4 a.

Next, a detection operation (measurement operation) carried out on theAC voltage V1 of the detected object 4 by the voltage detectingapparatus 101 will now be described.

First, the floating circuit unit 2C (or the entire voltage detectingapparatus 101) is positioned near the detected object 4 so that thedetection electrode 22 faces but does not contact the detected object 4.By doing so, as shown in FIG. 10, the capacitance C0 is formed betweenthe detection electrode 22 and the detected object 4. Here, although thevalue of the capacitance C0 will change inversely proportionally to thedistance between the detection electrode 22 and the detected object 4,once the floating circuit unit 2C has been initially disposed, suchcapacitance C0 will be a constant (i.e., non-varying) value so long asthe environmental conditions, such as temperature, are constant. Thevalue of the capacitance C0 is normally in a range of several pF toseveral tens of pF or so.

Next, when the voltage detecting apparatus 101 is started, if thepotential difference Vdi between the AC voltage V1 of the detectedobject 4 and the voltage of the guard electrode 21 (the referencevoltage or voltage of the voltage signal V4) has increased (for examplewhen the potential difference Vdi has increased due to an increase inthe AC voltage V1), the bootstrap circuit 27 increases the voltage valueof the detection voltage signal V2 outputted by the bootstrap circuit 27in accordance with the increase in the potential difference Vdi. Also,in keeping with the voltage increase in the detection voltage signal V2,the transistor 25 b of the driving circuit 25 makes a transition to adeeper ON state. By doing so, in the photocoupler 26, the currentflowing in the light-emitting diode increases and the resistance of thephototransistor falls. Accordingly, the voltage of the detection voltagesignal V2 a generated by dividing the potential difference (Vdd−Vss)between the resistance of the resistor 33 and the resistance of thephototransistor falls. The voltage generating circuit 34 increases thevoltage of the voltage signal V4 generated by the voltage generatingcircuit 34 based on the detection voltage signal V2 a. In the voltagedetecting apparatus 101, the bootstrap circuit 27, the driving circuit25, the photocoupler 26, and the main circuit unit 3 that construct afeedback loop in this way detect the increase in the AC voltage V1 ofthe detected object 4, and by carrying out a feedback control operationthat increases the voltage value of the voltage signal V4, cause thevoltage of the guard electrode 21 (i.e., the voltage of the voltagesignal V4) to follow the AC voltage V1.

When the potential difference Vdi has increased due to a drop in the ACvoltage V1, the bootstrap circuit 27 and the like that construct thefeedback loop carry out the opposite operation to the feedback controloperation described above to lower the voltage of the voltage signal V4and thereby cause the voltage of the guard electrode 21 (i.e., thevoltage of the voltage signal V4) to follow the AC voltage V1. By doingso, in the voltage detecting apparatus 101, a feedback control operationthat causes the voltage of the guard electrode 21 (i.e., the voltage ofthe voltage signal V4) to follow the AC voltage V1 is carried out in ashort time, resulting in the voltage of the guard electrode 21 (whichdue to virtual shorting of the operational amplifier 27 a, is also thevoltage of the detection electrode 22) being set equal to the AC voltageV1. The voltmeter 35 measures (detects) and displays the effective valueof the voltage signal V4 (the reference voltage, or voltage of the guardelectrode 21) in real time. Accordingly, by observing the numberdisplayed by the voltmeter 35, the operator can detect (measure) the ACvoltage V1 of the detected object 4.

In this way, in the voltage detecting apparatus 101, in a state wherethe detection electrode 22 has been disposed facing the detected object4, the bootstrap circuit 27 generates, based on the potential differenceVdi between the AC voltage V1 and the voltage signal (reference voltage)V4, the detection voltage signal (detection signal) V2 whose amplitudechanges in accordance with the potential difference Vdi, thephotocoupler 26 converts the detection voltage signal V2 to thedetection voltage signal (insulated detection signal) V2 a that iselectrically insulated from the detection voltage signal V2, and thevoltage generating circuit 34 generates the voltage signal V4 based onthe detection voltage signal V2 a and applies the voltage signal V4 tothe guard electrode 21. In the voltage detecting apparatus 101, sincethe circuit connected to the detection electrode 22 is the bootstrapcircuit 27 that has extremely high input impedance, the impedance of thedetection electrode 22 is maintained at a high impedance, and the entirefloating circuit unit 2C that is the load of the voltage generatingcircuit 34 will have a high impedance (a state where the load of thevoltage generating circuit 34 is light).

This means that according to the voltage detecting apparatus 101, thevoltage generating circuit 34 is capable of causing the voltage of thevoltage signal V4 outputted by the voltage generating circuit 34 tofavorably follow an AC voltage V1 with a short period (i.e., a highfrequency), and as a result, it is possible to precisely detect(measure) the AC voltage V1 across a wide frequency range (a broadfrequency band). That is, the voltage detecting apparatus 101 solves thesecond problem described above and therefore achieves the second objectdescribed above.

According to the voltage detecting apparatus 101, by using thephotocoupler 26 as the insulating circuit, it is possible to easilyelectrically insulate (i.e., separate) the floating circuit unit 2C andthe main circuit unit 3. Since the photocoupler 26 also has favorablefrequency characteristics across a wide frequency range, it is possibleto precisely detect (measure) the AC voltage V1 of the detected object 4across a wide range of frequencies. Note that it is also possible toconstruct the insulating circuit using a transformer (such as a pulsetransformer) in place of an optical insulating element such as thephotocoupler 26 and to construct the insulating circuit by connectingthe photocoupler 26 and a transformer in parallel. With suchconstructions, the primary coil of the transformer functions as theprimary-side circuit of the insulating circuit and the secondary coilfunctions as the secondary-side circuit. With the former construction,since a transformer will normally have favorable frequencycharacteristics at a higher frequency band than the photocoupler 26, byusing the transformer, it is possible to increase the maximum frequencyof the frequency band where detection of the AC voltage V1 is possible.With the latter construction, by having the photocoupler 26 operatemainly at low frequencies and the transformer operate mainly at highfrequencies, it is possible to provide the insulating circuit withfavorable frequency characteristics over a wide frequency band. As aresult, it is possible to detect (measure) the AC voltage V1 of thedetected object 4 precisely over a wider range of frequencies.

Also, according to the voltage detecting apparatus 101, by using aconstruction where the bootstrap circuit 27, the driving circuit 25, andthe photocoupler 26 are enclosed within the guard electrode 21 so as tobe covered by the guard electrode 21, it is possible to protect suchcircuits from the effects of external magnetic fields. As a result, itis possible to improve the detection precision of the AC voltage V1.

According to the voltage detecting apparatus 101, by disposing thedetection electrode 22 inside the guard electrode 21 at a position thatfaces the opening 21 a formed in the guard electrode 21 but does notprotrude from the opening 21 a (a “non-protruding state” for the presentinvention), it is possible to protect the detection electrode 22 fromthe effects of external magnetic fields and thereby significantlyimprove the detection precision of the AC voltage V1.

Also, according to the voltage detecting apparatus 101, by using aconstruction where the entire surface of the detection electrode 22 thatfaces the detected object 4 is covered with the insulating layer 21 b asan insulator, it is possible to reliably prevent shorting between thedetected object 4 and the detection electrode 22.

It is also possible to apply the present invention to a constructionequipped, together with a construction where the voltmeter 35 detects(measures) the effective value of the voltage signal V4, or in place ofsuch construction, with a DSP (Digital Signal Processor) that samplesfor example the voltage signal V4 and generates the waveform data of thevoltage signal V4. By using this construction, it is possible to outputthe waveform data outside the voltage detecting apparatus 101 and/or todisplay a waveform of the AC voltage V1 on a display unit provided inthe voltage detecting apparatus 101 based on the waveform data.

Also, although the floating circuit unit 2C of the bootstrap circuit 27shown in FIG. 10 described above uses a construction where the resistor27 e is disposed between the inverted input terminal and the outputterminal of the operational amplifier 27 a to provide the operationalamplifier 27 a with a gain, this can also be realized by using theconstruction shown in FIG. 12 in place of such construction. In thebootstrap circuit 27 of the other floating circuit unit 2C shown in FIG.12, by shorting the inverted input terminal of the operational amplifier27 a and the output terminal of the operational amplifier 27 a withoutproviding the resistor 27 e, the operational amplifier 27 a is caused tofunction as a buffer (i.e., an amplifier with a gain of 1).

Also, in the same way as the line voltage detecting apparatus 51 (seeFIG. 9) described above that is constructed using a plurality of voltagedetecting apparatuses such as the voltage detecting apparatus 1, it ispossible to construct a line voltage detecting apparatus (a second linevoltage detecting apparatus) 151 such as that shown in FIG. 9 using aplurality of the voltage detecting apparatuses 101. Note that since theline voltage detecting apparatus 151 is constructed using a plurality ofvoltage detecting apparatuses 101 in place of the voltage detectingapparatuses 1 of the line voltage detecting apparatus 51, detaileddescription of the construction is omitted.

Next, a detection operation (measurement operation) by the line voltagedetecting apparatus 151 will be described.

First, during detection (measurement), as shown in FIG. 9, to detect(measure) the AC voltage Vrp of the path R using the voltage detectingapparatus 101 r, the floating circuit unit 2 of the voltage detectingapparatus 1 r is positioned near the path R and the detection electrode22 of the voltage detecting apparatus 1 r is positioned opposite thecorresponding path R. In the same way, to detect (measure) the ACvoltages Vsp, Vtp of the paths S, T, the detection electrodes 22 of therespective floating circuit units 2 of the other voltage detectingapparatuses 101 s, 101 t are positioned opposite the corresponding pathsS, T. By doing so, the capacitance C0 (see FIG. 1) is formed betweeneach of the respective detection electrodes 22 and the paths R, S, T.Here, although the respective values of the capacitance C0 will changeinversely proportionately to the distances between the respectivedetection electrodes 22 and the cores of the paths R, S, T, after thefloating circuit units 2 have been disposed, such capacitances C0 willbe constant (i.e., not vary) so long as environmental conditions such ashumidity are constant. Also, by connecting (shorting) the ground G1 ofeach voltage detecting apparatuses 101, the potential of the ground G1in each voltage detecting apparatus 101 is set equal.

When the line voltage detecting apparatus 151 is started, in the voltagedetecting apparatus 101 r, the bootstrap circuit 27, the driving circuit25, the photocoupler 26, and the main circuit unit 3 that construct thefeedback loop carry out a feedback control operation that changes thevoltage value of the voltage signal V4 in accordance with changes in theAC voltage Vrp of the path R to cause the voltage of the guard electrode21 (i.e., the voltage of the voltage signal V4 and also the voltage ofthe detection electrode 22) to follow the AC voltage Vrp. Similarly, ineach of the other voltage detecting apparatuses 101 s, 101 t, thebootstrap circuit 27, the integrating circuit 24, the driving circuit25, the photocoupler 26, and the main circuit unit 3 that construct thefeedback loops carry out a feedback control operation that changes thevoltages of the respective voltage signals V4 in accordance with changesin the AC voltages Vsp, Vtp of the paths S, T to cause the respectivevoltages of the guard electrodes 21 (i.e., the voltages of the voltagesignals V4 and also the voltages of the detection electrodes 22) tofollow the AC voltages Vsp, Vtp. The voltmeters 35 of the voltagedetecting apparatuses 101 continuously output voltage data Dva, Dvb, Dvcrespectively showing the waveforms of detected (measured) voltagesignals V4, that is, the AC voltages Vrp, Vsp, Vtp of the paths R, S, T.

The calculation unit 52 inputs the voltage data Dva, Dvb, Dvc outputtedfrom the respective voltage detecting apparatuses 101 and stores thedata in the memory. Next, the calculation unit 52 carries out a linevoltage calculating process. More specifically, the calculation unit 52calculates the voltage difference between the voltage data Dva, Dvb tofind (calculate) the line voltage Vrs between the paths R, S. In thesame way, the calculation unit 52 calculates the voltage differencebetween the voltage data Dvb, Dvc to find (calculate) the line voltageVst between the paths S, T and calculates the voltage difference betweenthe voltage data Dva, Dvc to find (detect) the line voltage Vrt betweenthe paths R, T. In this case, as described earlier, since the ACvoltages Vrp, Vsp, Vtp of the paths R, S, T are detected (measured) bythe respective voltage detecting apparatuses 101 with the shared groundG1 as a reference, by calculating the voltage differences between the ACvoltages Vrp, Vsp, Vtp, the line voltages Vrs, Vst, Vrt can beaccurately found (calculated) regardless of the potential of thereference potential. The calculation unit 52 also has the calculatedline voltages Vrs, Vst, Vrt displayed on the display unit 53.

In this way, according to the line voltage detecting apparatus 151, byusing the voltage detecting apparatuses 101 that each include thebootstrap circuit 27, it is possible to accurately detect the ACvoltages Vrp, Vsp, Vtp across a wide frequency band. This means it ispossible to also detect (measure) the line voltages Vrs, Vst, Vrt acrossa wide frequency band.

Note that although an example has been described where a plurality ofthe voltage detecting apparatuses 101 of the same construction that arerespectively equipped with the main power supply circuit 31 and theconverter 32 are used, it is also possible to apply the presentinvention to a construction where the main power supply circuit 31 andthe converter 32 are provided in one out of the plurality of voltagedetecting apparatuses 101, and the positive voltage Vdd, the negativevoltage Vss, the positive voltage Vf+, and the negative voltage Vf− aresupplied from such voltage detecting apparatus 101 to the other voltagedetecting apparatuses 101.

Third Embodiment

A third embodiment of a voltage detecting apparatus and a line voltagedetecting apparatus will now be described with reference to the attacheddrawings.

First, a voltage detecting apparatus (third voltage detecting apparatus)201 according to the present embodiment will be described with referenceto the drawings. Note that components that are the same as in thevoltage detecting apparatuses 1, 101 described above have been assignedthe same reference numerals and description thereof is omitted.

The voltage detecting apparatus 201 is a non-contact voltage detectingapparatus and as shown in FIG. 13 includes a floating circuit unit 2Dand a main circuit unit 3C. The voltage detecting apparatus 201 isconstructed so as to be capable of detecting the AC voltage V1 (detectedAC voltage) generated in the detected object 4 using ground potential Vgas a reference.

As shown in FIG. 13, the floating circuit unit 2D includes a guardelectrode 211, a detection electrode 212, a power supply unit 213, adetection unit 214, and an insulating unit 215. The guard electrode 211is constructed using a conductive material (as one example, a metalmaterial) as a reference voltage unit in the floating circuit unit 2D,and encloses the detection electrode 212, the detection unit 214, andthe insulating unit 215. Note that as described later, since theinsulating unit 215 is equipped with a function that outputs a signalinputted into the primary-side circuit thereof from the secondary-sidecircuit thereof so as to be electrically insulated from the primary-sidecircuit, the part of the floating circuit unit 2D that needs to becovered by the guard electrode 211 may extend as far as the primary-sidecircuit, though it is also possible to use a construction where thesecondary-side circuit is also covered by the guard electrode 211. Inthe present embodiment, as one example, an opening (hole) 211 a isformed in the guard electrode 211. The detection electrode 212 is formedin a plate-like shape, for example, and is disposed at a position facingthe opening 211 a inside the guard electrode 211 so as not to contactthe guard electrode 211. When detecting the AC voltage V1, the detectionelectrode 212 is capacitively coupled to the detected object 4 as shownin FIG. 13 (to produce the capacitance C0).

The power supply unit 213 is constructed as a floating power supply thatgenerates a variety of floating voltages that have the voltage Vr of theguard electrode 211 as a reference (zero volts). The power supply unit213 supplies the generated floating voltages as operation voltages tothe component elements disposed inside the guard electrode 211. In thepresent embodiment, as one example, the power supply unit 213 includes abattery and a DC/DC converter (neither of which is shown), and the DC/DCconverter generates the variety of floating voltages (for example, Vf+that is a plus voltage and Vf− that is a minus voltage) as the operationvoltages based on a DC voltage outputted from the battery. Note thatalthough not shown, in place of the battery, it is also possible to usea construction where the AC voltage is supplied, via a transformer fromoutside the guard electrode 211 to the inside of the guard electrode 211so as to be electrically insulated therefrom, and the AC voltage isconverted by a rectifying/smoothing unit provided inside the guardelectrode 211 to a DC voltage and supplied to the DC/DC converter.

The detection unit 214 operates by being supplied with the floatingvoltages Vf+, Vf− set with the voltage Vr of the guard electrode 211 asa reference and generates a detection signal S1 whose amplitude changesin accordance with the AC potential difference (V1−Vr) based on thecurrent signal I (detection current) that flows with a current valuethat corresponds to the AC potential difference (V1−Vr) between the ACvoltage V1 and the voltage Vr of the guard electrode 211. Here, astandard signal Ss is outputted (applied) from a standard signaloutputting unit 231, described later, via a capacitor C31 a to the guardelectrode 211 and a voltage signal S4 is also outputted (applied) from afeedback control unit 237, described later. With this construction, thevoltage Vr is a composite voltage composed of the voltage (feedbackvoltage) V4 of the voltage signal S4 and the voltage Vs of the standardsignal Ss. By doing so, the current signal I described above is composedof a current signal component (reference current component) Is1 due tothe standard signal Ss, a current signal component (FB currentcomponent) Ib1 due to the voltage signal S4, and a current signalcomponent (measured current component) Iv1 due to the AC voltage V1 ofthe detected object 4. The detection signal S1 based on the currentsignal I is also composed of a voltage signal component (standardvoltage component) Vs1 based on the standard current component Is1, avoltage signal component (FB voltage component) Vb1 based on an FBcurrent component Ib1, and a voltage signal component (measured voltagecomponent) Vv1 based on the measured current component Iv1. Also, sincethe detection unit 214 generates the detection signal S1 by operatingwith the voltage of the guard electrode 211 that varies due to thevoltage Vs of the standard signal Ss and the voltage V4 of the voltagesignal S4 as a reference, the standard voltage component Vs1 included inthe detection signal S1 is a signal with opposite phase to the standardsignal Ss and the FB voltage component Vb1 included in the detectionsignal S1 is a signal with opposite phase to the voltage signal S4.

In the present embodiment, as one example, as shown in FIG. 14, thedetection unit 214 includes an integrating circuit 221 and an amplifiercircuit 222. The integrating circuit 221 includes an operationalamplifier 221 a with a non-inverted input terminal connected to theguard electrode 211 and an inverted input terminal connected to thedetection electrode 212, a capacitor 221 b connected between theinverted input terminal and the output terminal of the operationalamplifier 221 a, and a resistor 221 c that is connected in parallel withthe capacitor 221 b. In this case, the capacitor 221 b is composed of acapacitor with a capacitance of around 0.01 μF for example, and theresistor 221 c is constructed of a resistor with a high resistance ofaround 1 MΩ, for example. For this reason, in the integrating circuit221, due to the current signal I mainly flowing in the capacitor 221 b,a current-to-voltage converting operation and an integrating operationare simultaneously carried out to generate a voltage signal S0 whosevoltage changes in proportion to the AC potential difference (V1−Vr)between the AC voltage V1 of the detected object 4 and the voltage Vr ofthe guard electrode 211. Note that in the integrating circuit 221, sincethere is the risk that if only the capacitor 221 b were used, the amountof feedback would become remarkably low in a state close to DC and thegain would become extremely high, resulting in saturation of theoperational amplifier 211 a by the offset produced by the bias current,the resistor 221 c is disposed so as to suppress the drop in dynamicrange due to such saturation. The amplifier circuit 222 amplifies thevoltage of the voltage signal S0 by a predetermined gain and outputs theresult as the detection signal S1. Note that although not shown, as oneexample the integrating circuit 221 may be constructed of two circuits,that is, a current-to-voltage converting circuit that converts thecurrent signal I to a voltage signal and an integrating circuit thatintegrates the voltage signal and outputs the result as the detectionsignal S1.

The insulating unit 215 inputs the detection signal S1 and outputs thedetection signal S1 as an insulated detection signal S2 that iselectrically insulated from the detection signal S1. More specifically,as one example, the insulating unit 215 is constructed using an opticalinsulating element (as one example in the present embodiment, aphotocoupler) and outputs the detection signal S1 inputted into alight-emitting diode (not shown) used as the primary-side circuit of theinsulating unit 215 as the insulated detection signal S2 from aphototransistor as the secondary-side circuit of the insulating unit215. That is, the insulating unit 215 outputs a signal that has the samephase as the detection signal S1 and whose amplitude changes inproportion to the amplitude of the detection signal S1 as the insulateddetection signal S2. Note that in place of a photocoupler, it is alsopossible to construct the insulating unit 215 using an optical MOS-FETconstructed with a light-emitting diode as the primary-side circuit anda FET pair as the secondary-side circuit. In this case, the primary-sidecircuit of the insulating unit 215 operates by being supplied with thefloating voltages Vf+, Vf−. When the detection signal S1 ishigh-frequency AC, it is also possible to construct the insulating unit215 using a transformer.

The floating circuit unit 2D constructed as described above has flatfrequency characteristics in a wide frequency band that extends from alow frequency (several Hz) to a high frequency (several hundred Hz). Asdescribed above, the floating circuit unit 2D detects the current signalI (detection current) that flows with a magnitude in keeping with thepotential difference (V1−Vr) and generates and outputs the insulateddetection signal S2 whose amplitude changes in accordance with the ACpotential difference (V1−Vr).

As shown in FIG. 13, the main circuit unit 3C includes the standardsignal outputting unit 231, a signal extracting unit 232, a processingunit 233, a storage unit 234, an output unit 235, an amplitudeconverting unit 236, and a feedback control unit 237. Here, the standardsignal outputting unit 231 generates the standard signal Ss (an Acsignal whose frequency and amplitude are constant) with a constantamplitude and a voltage Vs that changes with a predetermined period withthe ground potential Vg as a reference and outputs the standard signalSs via the capacitor 231 a to the guard electrode 211. As one example,in the present embodiment, as described later, the frequency fs of thestandard signal Ss is set within a frequency band W3 that exceeds thefrequency bands W1, W2 where the feedback control unit 237 is capable ofresponse (see FIG. 16). The amplitude converting unit 236 inputs thevoltage (the voltage Vr) generated in the guard electrode 211 as avoltage signal Sr, changes the amplitude thereof (by k times, where k isa positive real number) and outputs the result as a voltage signal(reference signal) Sr1. Here, as described earlier, since the voltage Vrincludes the voltage V4 of the voltage signal S4 and the voltage Vs ofthe standard signal Ss, the reference signal Sr1 is a composite voltagecomposed of the voltages V4, Vs that have been amplified by k times. Asone example, in the present embodiment, the amplitude converting unit236 is constructed of an attenuator (as one example, two resistors 236a, 236 b that are connected in series), changes the amplitude (in thepresent embodiment, reduces the amplitude) of the voltage signal Sr bydividing the voltage, and outputs the result as the reference signalSr1. Note that it is also possible to construct the amplitude convertingunit 236 of an amplifier that amplifies a signal by a predetermined gainto make the amplitude of the reference signal Sr1 larger than theamplitude of the voltage signal Sr.

As one example, the signal extracting unit 232 includes an amplifiercircuit 241, an adder circuit 242, a synchronous detection circuit 243,and a control circuit 244, and amplifies the insulated detection signalS2 by a predetermined gain to generate an amplified detection signal S3.Also, by controlling the gain when amplifying the insulated detectionsignal S2 so that the signal component of the standard signal Ssincluded in the amplified detection signal S3 (hereinafter also referredto as the “first signal component”) and the signal component of thestandard signal Ss included in the reference signal Sr1 (hereinafteralso referred to as the “second signal component”) will cancel eachother out when the amplified detection signal S3 and the referencesignal Sr1 are subjected to addition or subtraction (addition as oneexample in the present embodiment), the signal extracting unit 232generates and outputs an output signal So composed of a signal componentof the AC voltage V1 as described later. Here, the first signalcomponent of the standard signal Ss included in the amplified detectionsignal S3 refers to a signal component due to the standard voltagecomponent Vs1 included in the detection signal S1 (that is, a signalcomponent with the same frequency as the standard signal Ss included inthe amplified detection signal S3) that is caused by the outputting(application) of the standard signal Ss to the guard electrode 211. Thesecond signal component of the standard signal Ss included in thereference signal Sr1 refers to a signal component with the samefrequency as the standard signal Ss included in the reference signal Sr1that is caused by the outputting (application) of the standard signal Ssto the guard electrode 211.

More specifically, the amplifier circuit 241 inputs the insulateddetection signal S2 and generates and outputs the amplified detectionsignal S3 by amplifying the insulated detection signal S2 by a gain (again that may be one or greater, or under one) set by the level (DCvoltage level) of a control signal (more specifically, control voltage)Sc outputted from the control circuit 244. As one example, as shown inFIG. 15, the amplifier circuit 241 includes an operational amplifier 241a, a variable resistance element (as one example in the presentembodiment, a J-FET (Junction Field Effect Transistor)) 241 b disposedbetween an inverted input terminal and a ground terminal of theoperational amplifier 241 a, and a resistor 241 c that is disposedbetween the inverted input terminal and the output terminal of theoperational amplifier 241 a, and is constructed as a whole as anon-inverting amplifier circuit. Here, the variable resistance element241 b changes its resistance in accordance with the level of theinputted control signal Sc. This means that the amplifier circuit 241changes its gain in accordance with the level of the inputted controlsignal Sc, amplifies the insulated detection signal S2 by such gain, andoutputs the result as the amplified detection signal S3. Note that asthe variable resistance element, it is also possible to use an elementor circuit aside from the J-FET so long as such element or circuitchanges its resistance in accordance with a voltage inputted fromoutside. In the present embodiment, as one example, the variableresistance element 241 b is constructed so that the resistance thereoffalls when the level of the inputted control signal Sc has increased andthe resistance thereof rises when the level of the inputted controlsignal Sc has decreased. With this construction, the gain of theamplifier circuit 241 will increase when the level of the control signalSc has increased and will decrease when the level of the control signalSc has decreased.

The adder circuit 242 inputs the amplified detection signal S3 and thereference signal Sr1 and adds the signals S3, Sr1 and outputs the addedsignal obtained by the addition as the output signal So. Here, asdescribed above, the detection signal S1 is composed of the standardvoltage component Vs1 with opposite phase to the standard signal Ss, theFB voltage component Vb1 with opposite phase to the voltage signal S4,and the measured voltage component Vv1 with the same phase as the ACvoltage V1. This means that the insulated detection signal S2 generatedbased on the detection signal S1 and the amplified detection signal S3generated by amplifying the insulated detection signal S2 are composedof a signal component with opposite phase to the standard signal Ss, asignal component with opposite phase to the voltage signal S4, and asignal component with the same phase as the AC voltage V1. In this case,the amplified detection signal S3 is controlled so that the amplitude ofthe first signal component (hereinafter also referred to as the“opposite-phase signal component”) that has the opposite phase to thestandard signal Ss included in the amplified detection signal S3 isfinally set at the same amplitude as the amplitude of the second signalcomponent of the standard signal Ss included in the reference signal Sr1outputted from the amplitude converting unit 236 (i.e., an amplitudeproduced by multiplying the amplitude of the standard signal Ss by k:k×Ss).

On the other hand, since the voltage Vs of the voltage signal Sr is acomposite voltage composed of the voltage V4 of the voltage signal S4and the voltage Vs of the standard signal Ss as described above, thereference signal Sr1 that is generated by multiplying the amplitude ofthe voltage signal Sr by k is composed of a signal component with thesame phase as the standard signal Ss (a signal produced by multiplyingthe amplitude of the standard signal Ss by k) and a signal componentwith the same phase as the voltage signal S4 (a signal produced bymultiplying the amplitude of the voltage signal S4 by k).

Accordingly, by subjecting both signals S3, Sr1 to an addition processusing the adder circuit 242, the opposite-phase signal component (“firstsignal component”) of the standard signal Ss that constructs theamplified detection signal S3 and a second signal component (hereinafterreferred to as the “same-phase signal component”) with the same phase asthe standard signal Ss that constructs the reference signal Sr1 canceleach other out. This means that the output signal So includes (i) twosignal components that construct the amplified detection signal S3 andare a signal component with the opposite phase to the voltage signal S4and a signal component with the same phase as the AC voltage V1 and (ii)a signal component (i.e., a signal produced by multiplying the amplitudeof the voltage signal S4 by k) that constructs the reference signal Sr1and has the same phase as the voltage signal S4.

The synchronous detection circuit 243 inputs the output signal So andthe standard signal Ss and by carrying out synchronous detection on theoutput signal So using the standard signal Ss, generates and outputs awave detection signal Vd. More specifically, by carrying out synchronousdetection, the synchronous detection circuit 243 generates and outputsthe wave detection signal Vd where the absolute value of the voltageincreases or decreases in accordance with increases and decreases in theamplitude of the signal component of the standard signal Ss included inthe output signal So (more specifically, a signal component with thesame frequency as the standard signal Ss) and where the polarity differsaccording to whether the phase of the signal component of the standardsignal Ss included in the output signal So matches the phase of thesignal component of the standard signal Ss included in the output signalSo (i.e., when the phases match) or whether such phases are 180° apart(i.e., when the phases are opposite). In the present embodiment, as oneexample, the synchronous detection circuit 243 generates and outputs thewave detection signal Vd that has positive polarity (i.e., is a positivevoltage) when the predetermined signal component included in the outputsignal So and the standard signal Ss have the same phase and negativepolarity (i.e., is a negative voltage) when the phases are opposite.

The control circuit 244 generates the control signal Sc whose voltageincreases and decreases based on the polarity of the inputted wavedetection signal Vd and outputs the control signal Sc to the amplifiercircuit 241. In the present embodiment, as one example, the controlcircuit 244 raises the voltage level of the control signal Sc when theinputted wave detection signal Vd has positive polarity and converselylowers the voltage level of the control signal Sc when the inputted wavedetection signal Vd has negative polarity. With the above construction,in the signal extracting unit 232, feedback control is carried out bythe synchronous detection circuit 243 and the control circuit 244 overthe gain (amplification ratio) of the amplifier circuit 241, and thecontrol circuit 244 carries out control based on the wave detectionsignal Vd of the amplification ratio of the amplifier circuit 241 sothat the amplitude of the opposite-phase signal component thatconstructs the amplified detection signal S3 (i.e., the first signalcomponent with the same frequency as but opposite phase to the standardsignal Ss) becomes a certain amplitude (in the present embodiment, sothat the amplitude is equal to the amplitude of the same-phase signalcomponent that constructs the reference signal Sr1 inputted into theadder circuit 242 (i.e., the second signal component with the samefrequency and the same phase as the standard signal Ss)). By doing so,the amplitude of the opposite-phase signal component that constructs theamplified detection signal S3 is set equal to the amplitude of thesame-phase signal component of the reference signal Sr1 inputted intothe adder circuit 242. Accordingly, as described above, the addercircuit 242 generates and outputs the output signal So composed of asignal component with the opposite phase to the voltage signal S4, asignal component with the same phase as the AC voltage V1 (the twosignal components mentioned above that construct the amplified detectionsignal S3), and the signal component with the same phase as the voltagesignal S4 (the signal component mentioned above that constructs thereference signal Sr1).

Here, in accordance with the magnitude of the capacitance C0 formedbetween the detected object 4 and the detection electrode 212, thestandard current component Is1 and the measured current component Iv1included in the current signal I will vary with the same proportions andthe standard voltage component Vs1 and the measured voltage componentVv1 included in the detection signal S1 will also vary with the sameproportions. Accordingly, although both the opposite-phase signalcomponent that constructs the amplified detection signal S3 (a signalcomponent with the same frequency as the standard signal Ss) and thesignal component with the same frequency as the AC voltage V1 will varywith the same proportions, in the signal extracting unit 232, due to thefeedback control described above, the amplified detection signal S3 isgenerated by the amplifier circuit 241 so that the amplitude of theopposite-phase signal component (first signal component) that constructsthe signal S3 matches the amplitude of the same-phase signal component(second signal component) that constructs the reference signal Sr1. Forthis reason, in the construction of the present embodiment, the voltagecomponents based on the measured current component Iv1 included in theoutput signal So, that is, the signal components that construct theamplified detection signal S3 (the signal component with opposite phaseto the voltage signal S4 and the signal component with the same phase asthe AC voltage V1) will have an amplitude whose magnitude corresponds tothe difference between the AC voltage V1 generated by the detectedobject 4 and the voltage signal S4 regardless of the magnitude of thecapacitance C0. The signal component that has the same phase as thevoltage signal V4 that constructs the reference signal Sr1 included inthe output signal So is fundamentally a signal component that isgenerated irrespective of the magnitude of the capacitance C0.Accordingly, the output signal So is a signal that is unaffected by themagnitude of the capacitance C0.

The feedback control unit (voltage generating circuit) 237 inputs andamplifies the insulated detection signal S2 to generate the voltagesignal S4 of the voltage V4 (feedback voltage), and outputs (applies)the voltage signal S4 to the guard electrode 211. Here, together withthe guard electrode 211, the detection electrode 212, the detection unit214, and the insulating unit 215 of the floating circuit unit 2D, thefeedback control unit 237 forms a feedback loop and generates thevoltage signal S4 by carrying out an amplification operation thatamplifies the insulated detection signal S2 so that the potentialdifference Vdi between the AC voltage V1 and the voltage Vr of the guardelectrode 211 falls. In the present embodiment, as one example, thefeedback control unit 237 includes an AC amplifier circuit 237 a, aphase compensating circuit 237 b, and a booster circuit 237 c. Here, theAC amplifier circuit 237 a inputs and amplifies the insulated detectionsignal S2 to generate the voltage signal V4 a. In this case, the ACamplifier circuit 237 a carries out an amplification operation on thevoltage signal V4 a so that the absolute value of the voltage thereofchanges in accordance with increases and decreases in the absolute valueof the voltage of the insulated detection signal S2.

To stabilize the feedback control operation (by preventing vibration),the phase compensating circuit 237 b inputs the voltage signal V4 a,adjusts the phase of the voltage signal V4 a, and outputs the result asthe voltage signal V4 b. The booster circuit 237 c is constructed usinga boosting transformer, for example, and by boosting the voltage signalV4 b by a predetermined gain (i.e., by increasing the absolute valuewithout changing the polarity), generates the voltage signal S4, andoutputs the voltage signal S4 to the guard electrode 211. Also, theoutput impedance of the booster circuit 237 c is set at a highimpedance. The feedback control unit 237 constructed in this waygenerates and outputs the voltage signal S4 whose amplitude changes withthe frequency characteristics shown in FIG. 16. Due to these frequencycharacteristics, for the case of a signal (the AC voltage V1) of afrequency in the low frequency band W1 out of the frequency bands W1, W2covered by the response of the feedback control unit 237, the feedbackcontrol unit 237 will favorably follow the signal and will thereforegenerate and output a voltage signal S4 of the same voltage V4 as the ACvoltage V1. However, for the case of a signal (the AC voltage V1) of afrequency included in the high frequency band W2 out of the frequencybands W1, W2 covered by the response of the feedback control unit 237,due to insufficient gain, the feedback control unit 237 will generateand output a voltage signal S4 of the voltage V4 that is below the ACvoltage V1. The feedback control unit 237 will also generate and outputa voltage signal S4 whose voltage V4 is substantially zero volts forsignals (including the standard signal Ss) in a frequency band W3 thatexceeds the frequency band W2 without following such signals.

The processing unit 233 includes an A/D converter and a CPU (neither ofwhich is shown), and carries out a storage process that samples thevoltage waveform (level) of the output signal So using a sampling clockof a predetermined frequency to convert the waveform to digital data D1and stores the digital data D1 in the storage unit 234, a voltagecalculating process that calculates the AC voltage V1 based on thedigital data D1, and an output process that outputs the calculated ACvoltage V1. The storage unit 234 is constructed of a ROM, a RAM, or thelike, and stores in advance a voltage calculation table TB used in thevoltage calculating process carried out by the processing unit 233. Anoverview of a generation procedure for the voltage calculation table TBwill now be described. As one example, in a state where a standardsignal Ss with a known voltage Vs (a certain voltage) is being outputtedto the guard electrode 211 and feedback control is being carried out bythe synchronous detection circuit 243 and the control circuit 244, theamplitude of the AC voltage V1 generated in the detected object 4 isconverted in predetermined voltage steps to obtain the digital data D1,and in association with the AC voltage V1 that changes in the voltagesteps, the digital data D1 is stored together with the voltage value ofthe AC voltage V1 to generate the voltage calculation table TB. Withthis construction, by referring to the voltage calculation table TB toobtain the voltage value of the AC voltage V1 that corresponds to theobtained digital data D1, the processing unit 233 can calculate the ACvoltage V1 of the detected object 4. As one example in the presentembodiment, the output unit 235 is constructed of a display apparatus,and in the output process carried out by the processing unit 233,displays the waveform of the AC voltage V1 and/or calculated voltageparameters (amplitude, effective value, or the like).

Next, a detection operation carried out on the AC voltage V1 of thedetected object 4 by the voltage detecting apparatus 201 will now bedescribed.

First, the floating circuit unit 2D (or the entire voltage detectingapparatus 201) is positioned near the detected object 4 so that thedetection electrode 22 faces but does not contact the detected object 4.By doing so, as shown in FIG. 13, the capacitance C0 is formed betweenthe detection electrode 212 and the detected object 4. Here, althoughthe value of the capacitance C0 will change inversely proportionally tothe distance between the detection electrode 212 and the detected object4, once the floating circuit unit 2D has been initially disposed, suchcapacitance C0 will be a constant (i.e., non-varying) value so long asthe environmental conditions, such as temperature, are constant. Sincethe value of the capacitance C0 is normally extremely small (forexample, a range of several pF to around several tens of pF or so), evenif the frequency of the AC voltage V1 is around several hundred Hz, theimpedance between the detected object 4 and the detection electrode 212will be sufficiently large (several MΩ). For this reason, in the voltagedetecting apparatus 201, even when the AC voltage V1 of the detectedobject 4 and the voltage Vr of the guard electrode 211 greatly differ(i.e., when the potential difference Vdi is large), it will still bepossible to use a low-cost component with a low input withstand voltagein the operational amplifier 221 a that constructs the detection unit214. Even when this construction is used, breakdown of the operationalamplifier 221 a due to the potential difference Vdi is avoided.

By producing an AC connection between the detection electrode 212 andthe detected object 4 via the capacitance C0, a current path A (i.e.,the current path shown by the dot-dash line in FIG. 13) from the groundpotential Vg via the detected object 4, the detection electrode 212, thedetection unit 214, the guard electrode 211, the capacitor 231 a, thestandard signal outputting unit 231, and the feedback control unit 237,to the ground potential Vg is formed. This means that when the floatingcircuit unit 2D and the main circuit unit 3C are operating, a currentsignal I composed of the standard current component Is1 due to thevoltage Vs of the standard signal Ss, the measured current component Iv1due to the AC voltage V1 of the detected object 4 and the FB currentcomponent Ib1 due to the voltage V4 of the voltage signal S4 outputtedfrom the feedback control unit 237 to the guard electrode 211 will flowon the current path A.

With this construction, in the floating circuit unit 2D, as shown inFIGS. 13, 14, the integrating circuit 221 of the detection unit 214integrates the current signal I to generate the voltage signal S0 andthe amplifier circuit 222 amplifies the voltage signal S0 and outputsthe result as the detection signal S1. The insulating unit 215 inputsthe detection signal S1 and outputs the insulated detection signal S2that is electrically insulated from the detection signal S1.

In the main circuit unit 3C, the feedback control unit 237 generates thevoltage signal S4 based on the insulated detection signal S2 and outputsthe voltage signal S4 to the guard electrode 211. In this case, thefeedback control unit 237 generates the voltage signal S4 whoseamplitude changes with the frequency characteristics shown in FIG. 16(that is, the voltage signal S4 whose amplitude is the same as the ACvoltage V1 in the low frequency band W1, whose amplitude is virtuallyzero in the high frequency band W3, and whose amplitude graduallydecreases from the same state as the AC voltage V1 toward zero as thefrequency rises in an intermediate frequency band W2) and outputs thevoltage signal S4 to the guard electrode 211. The amplitude convertingunit 236 inputs the voltage Vr (a composite voltage composed of thevoltage V4 of the voltage signal S4 and the voltage Vs of the standardsignal Ss) generated in the guard electrode 211 as the voltage signalSr, changes the amplitude of the voltage Vr (by k times), and outputsthe result as the reference signal Sr1 with the frequencycharacteristics shown in FIG. 17.

Since the feedback control unit 237 operates as described above andgenerates the voltage signal S4 with the frequency characteristics shownin FIG. 16 and outputs the voltage signal S4 to the guard electrode 211,the insulated detection signal S2 generated by the floating circuit unit2D as described above based on the potential difference Vdi between theAC voltage V1 and such voltage signal S4 will be a signal where thesignal components for the AC voltage V1 and the voltage signal S4 (i.e.,components with the same frequency as the AC voltage V1) have anamplitude that changes with opposite frequency characteristics (see FIG.18) to the frequency characteristics of the voltage signal S4 (see FIG.16. That is, as shown in FIG. 18, the floating circuit unit 2D generatesand outputs the insulated detection signal S2 so that in the lowfrequency band W1, since the feedback control is carried out to make thevoltage V4 of the voltage signal S4 equal to the AC voltage V1 andthereby make the potential difference Vdi zero, the amplitude of theinsulated detection signal S2 becomes zero, in the high frequency bandW3, since the voltage V4 of the voltage signal S4 becomes substantiallyzero and the potential difference Vdi becomes equal to the AC voltageV1, the insulated detection signal S2 has an amplitude in proportion tothe AC voltage V1, and in the intermediate frequency band W2, theinsulated detection signal S2 has an amplitude that gradually increasesin keeping with a rise in frequency from a zero state toward theamplitude in the frequency band W3.

As described earlier, in the signal extracting unit 232, feedbackcontrol is carried out by the synchronous detection circuit 243 and thecontrol circuit 244 on the gain (amplification ratio) of the amplifiercircuit 241 and the control circuit 244 controls the gain of theamplifier circuit 241 based on the wave detection signal Vd so that theamplitude of the opposite-phase signal component that constructs theamplified detection signal S3 (the first signal component that has thesame frequency as but the opposite phase to the standard signal Ss)becomes a certain amplitude (i.e., in the present embodiment, so as tohave the same amplitude as the amplitude of the same-phase signalcomponent that constructs the reference signal Sr1 inputted into theadder circuit 242 (the second signal component that has the samefrequency and the same phase as the standard signal Ss)). By doing so,the amplifier circuit 241 generates and outputs the amplified detectionsignal S3 that has the frequency characteristics shown in FIG. 19 andwhere the amplitude of the opposite-phase signal component matches theamplitude of the same-phase signal component of the reference signal Sr1inputted into the adder circuit 242. In this case, as shown in FIG. 19,the amplitude of the amplified detection signal S3 in the frequency bandW3 is k times the AC voltage V1, which as shown in FIG. 17 matches theamplitude (i.e., k times the AC voltage V1) of the reference signal Sr1in the frequency band W1.

Accordingly, as shown in FIG. 20, by adding the amplified detectionsignal S3 (a signal composed of a signal component with opposite phaseto the voltage signal S4 and a signal component with the same phase asthe AC voltage V1) with the frequency characteristics shown in FIG. 19described above (the characteristics shown by the narrow solid line inFIG. 20) to the reference signal Sr1 (a signal composed of a signalcomponent with the same phase as the voltage signal S4) with thefrequency characteristics shown in FIG. 17 described above (thecharacteristics shown by the dot-dash line in FIG. 20), the addercircuit 242 generates and outputs the output signal So (a signal whoseamplitude is k times the amplitude of the AC voltage V1 across a wideband) that is composed of only a signal component with the same phase asthe AC voltage V1 and has flat frequency characteristics (thecharacteristics shown by the thick solid line in FIG. 20) across a widefrequency band from the low frequency band W1 to the high frequency bandW3. In this case, as shown by the broken line in FIG. 20, the signalcomponents of the standard signal Ss included in the amplified detectionsignal S3 and the reference signal Sr1 have matching amplitudes andtherefore cancel each other out.

Next, the processing unit 233 carries out the storage process to inputthe output signal So, convert the output signal So to the digital dataD1, and store the digital data D1 in the storage unit 234. After this,the processing unit 233 carries out the voltage calculating process. Inthis voltage calculating process, the processing unit 233 reads thedigital data D1 stored in the storage unit 234 and refers to the voltagecalculation table TB to obtain the AC voltage V1 corresponding to theread digital data D1. Also, the processing unit 233 calculates theeffective value, amplitude, and the like of the AC voltage V1 based onthe obtained AC voltage V1 and stores such information in the storageunit 234. The processing unit 233 finally carries out the output processand causes the output unit 235, which is constructed of a displayapparatus, to display the effective value, amplitude, and the like ofthe AC voltage V1 that are stored in the storage unit 234. By doing so,the detection of the AC voltage V1 of the detected object 4 by thevoltage detecting apparatus 201 is completed. Note that it is alsopossible to use a construction where in the output process, theprocessing unit 233 causes the output unit 235 to display the voltagewaveform of the AC voltage V1 based on the obtained AC voltage V1.

In the voltage detecting apparatus 201, the standard signal outputtingunit 231 outputs the standard signal Ss to the guard electrode 211, thedetection unit 214 that operates on being supplied with the floatingvoltage (the plus voltage Vf+ and the minus voltage Vf−) outputs thedetection signal S1 whose amplitude changes in accordance with the ACpotential difference (V1−Vr) based on the current signal I flowing witha current value corresponding to the AC potential difference (V1−Vr)between the AC voltage V1 and the voltage Vr of the guard electrode 211between the detected object 4 and the guard electrode 211 via thedetection electrode 212, and the insulating unit 215 inputs thedetection signal S1 and outputs the detection signal S1 as the insulateddetection signal S2. Based on the insulated detection signal S2, thefeedback control unit 237 generates the voltage signal S4 (with thevoltage V4) so as to follow the AC voltage V1 and outputs the voltagesignal S4 to the guard electrode 211. The signal extracting unit 232controls the gain for the insulated detection signal S2, changes theamplitude of the amplified detection signal S3 and outputs the amplifieddetection signal S3 so that the amplitude of the first signal componentof the standard signal Ss included in the amplified detection signal S3matches the amplitude of the second signal component of the standardsignal Ss included in the reference signal Sr1, and the signalextracting unit 232 adds the amplified detection signal S3 whoseamplitude has been changed in this way and the reference signal Sr1. Bydoing so, the signal extracting unit 232 generates the signal componentof the AC voltage V1 from which the signal components of the standardsignal Ss (i.e., signal components with the same frequency as thestandard signal Ss) have been removed and outputs such signal componentas the output signal So, and the processing unit 233 detects andcalculates the AC voltage V1 based on the level of the output signal Socomposed of the signal component of the AC voltage V1.

According to the voltage detecting apparatus 201, since the AC voltageV1 in a high frequency band that cannot be detected by the detectionoperation by the feedback control unit 237 alone can be detected basedon the amplified detection signal S3 generated by the signal extractingunit 232, it is possible to detect the AC voltage V1 in a non-contactmanner over a wide frequency band. That is, the voltage detectingapparatus 201 solves the third problem described above and thereforeachieves the third object described above. Also, according to thevoltage detecting apparatus 201, since the output signal So can bedetected as a signal that is unaffected by the coupling capacitance (thecapacitance C0) between the detected object 4 and the detectionelectrode 212, it is possible to detect the AC voltage V1 in anon-contact manner without calculating the capacitance C0.

Also, in the voltage detecting apparatus 201, in the signal extractingunit 232, the synchronous detection circuit 243 detects the wavedetection signal Vd showing the amplitude of the signal component of thestandard signal Ss included in the amplified detection signal S3 or theoutput signal So by carrying out synchronous detection using thestandard signal Ss and the control circuit 244 controls the gain of theamplifier circuit 241 based on the wave detection signal Vd. Therefore,according to the voltage detecting apparatus 201, it is possible toaccurately detect the signal component of the standard signal Ss bycarrying out synchronous detection, and by doing so, it is possible toset the amplitude of the first signal component of the standard signalSs included in the amplified detection signal S3 equal to the amplitudeof the second signal component of the standard signal Ss included in thereference signal Sr1 with high precision. As a result, it is possible tohave the first signal component and the second signal component cancelone another out with high precision, and since this greatly reduces thesignal component of the standard signal Ss included in the output signalSo, it becomes possible to significantly improve the detection precisionof the AC voltage V1.

Also, in the voltage detecting apparatus 201, the signal extracting unit232 includes, as a canceling circuit, the adder circuit 242 that carriesout a process that cancels out the first signal component(opposite-phase signal component) that constructs the amplifieddetection signal S3 and has opposite phase to the standard signal Ss andthe second signal component that is included in the reference signal Sr1and the same phase as the standard signal Ss, and the control circuit244 controls the gain of the amplifier circuit 241 so that the firstsignal component included in the amplified detection signal S3 inputtedinto the adder circuit 242 can be cancelled out by the second signalcomponent included in the reference signal Sr1. Therefore according tothe voltage detecting apparatus 201, since it is possible to construct acanceling circuit from a simple circuit such as an adder circuit, it ispossible to reliably generate the output signal So while simplifying theapparatus construction.

Also according to the voltage detecting apparatus 201, by including theprocessing unit 233 that detects the AC voltage V1 based on the outputsignal So, it is possible to cause the processing unit 233 to detect theAC voltage V1 at fixed intervals and store and save the detected ACvoltages V1 in the storage unit 234 and to cause the output unit 235 todisplay the voltage waveform of the AC voltage V1 based on the ACvoltages V1 stored in the storage unit 234.

Also, according to the voltage detecting apparatus 201, since theprocessing unit 233 calculates the voltage value of the AC voltage V1based on the output signal So, it is possible to detect (measure) the ACvoltage V1.

According to the voltage detecting apparatus 201, by changing theamplitude k times using the amplitude converting unit 236, it ispossible to increase the range of the AC voltage V1 that can bedetected. For example, even when the processing unit 233 has a ratedinput level for the output signal So (for a construction equipped withan A/D converter as described above, when the input level of the outputsignal So is limited to a predetermined level or below by the ratedinput of the A/D converter), by setting the multiplier k at thenumerical value 1/10, compared to the numerical value 1 (i.e., aconstruction where the standard signal Ss is directly inputted into theadder circuit 242), it is possible to detect (measure) an AC voltage V1of a higher voltage while still satisfying the rated input level for theoutput signal So.

Note that although the voltage detecting apparatus 201 described aboveis constructed to use the fact that the first signal component for thestandard signal Ss included in the amplified detection signal S3 hasopposite phase to the second signal component for the standard signal Ssincluded in the reference signal Sr1 and to use the adder circuit 242 asa canceling circuit so that the first signal component included in theamplified detection signal S3 and the second signal component includedin the reference signal Sr1 cancel each other out, it is also possibleto cause the detection unit 214, the insulating unit 215, and theamplifier circuit 241 to invert the phase of the detection signal S1,the insulated detection signal S2, and the amplified detection signal S3and/or to invert the phase of the reference signal Sr1 outputted to thecanceling circuit. With this construction, since it is possible to makethe phase of the first and second signal components for the standardsignal Ss inputted into the adder circuit 242 equal, by using asubtractor circuit as the canceling circuit, it is possible to cancelout the first signal component included in the amplified detectionsignal S3 and the second signal component included in the referencesignal Sr1.

Also, in the example described above, although a construction is usedwhere the voltage Vr of the guard electrode 211 is set as the voltagesignal Sr and the amplitude of the voltage signal Sr is changed by theamplitude converting unit 236 and inputted into the canceling circuit(in the above example, the adder circuit 242) as the reference signalSr1, it is also possible to apply the present invention to aconstruction where the amplitude converting unit 236 is omitted and thevoltage signal Sr is inputted into the canceling circuit as thereference signal. With this construction, the signal extracting unit 232carries out feedback control to make the amplitude of the first signalcomponent of the standard signal Ss included in the amplified detectionsignal S3 match the amplitude of the second signal component of thestandard signal Ss included in the voltage signal Sr as the referencesignal. Accordingly, in this construction, the amplitude of the voltagecomponent is detected in a state one time the amplitude (that is, anequal voltage to the AC voltage V1) based on the measured currentcomponent Iv1 included in the output signal So. This means theprocessing unit 233 is capable of directly detecting the AC voltage V1from the voltage component based on the detected measured currentcomponent Iv1.

Also, in the voltage detecting apparatus 201 described above, although aconstruction is used where the insulating unit 215 and the amplifiercircuit 241 are directly interconnected, the standard signal outputtingunit 231 and the adder circuit 242 are directly interconnected, and thestandard signal outputting unit 231 and the synchronous detectioncircuit 243 are directly interconnected, though not shown, it is alsopossible to provide a buffer between such components as necessary. Also,although an example has been described above where the standard signalSs outputted from the standard signal outputting unit 231 is supplied tothe synchronous detection circuit 243 with its level unchanged, thoughnot shown, it is also possible to use an attenuator constructed ofvoltage-dividing resistors, for example, to lower the standard signal Ssto a required level and supply the result to the synchronous detectioncircuit 243.

Also, although not shown, by equipping inside the main circuit unit 3Cwith an A/D converting unit that converts the insulated detection signalS2, which is an analog signal, to digital data and an A/D convertingunit that converts the standard signal Ss, which is also an analogsignal, supplied from the standard signal outputting unit 231 to thesignal extracting unit 232, it is possible to carry out all of theprocessing by the signal extracting unit 232 as digital processing. Insuch case, it is also possible to use a construction where theprocessing unit 233 is equipped with the functions of the signalextracting unit 232, and by using this construction, it is possible togreatly reduce the number of circuit components. The functions of theprocessing unit 233 and the functions of the signal extracting unit 232may be realized by software, or may be realized by hardware (such as aDSP (Digital Signal Processor) or a logic array).

Next, a line voltage detecting apparatus (a third line voltage detectingapparatus) 251 that uses a plurality of the voltage detectingapparatuses 201 described above will be described.

First, the line voltage detecting apparatus 251 will be described withreference to the drawings. Note that an example where line voltages of athree-phase (R phase, S phase, and T phase), three-wire AC paths(hereinafter simply “paths”) R, S, T are detected will be described.

As one example, as shown in FIG. 21, the line voltage detectingapparatus 251 includes an equal number of (here, three) voltagedetecting apparatuses 201 to the number of paths R, S, T (hereinafter,the voltage detecting apparatuses 201 are referred to individually asthe voltage detecting apparatuses 201 r, 201 s, 201 t corresponding tothe paths R, S, T and as the “voltage detecting apparatuses 201” when nospecial distinction is required), a calculation unit 252, and a displayunit 253, and is constructed so as to be capable of detecting a linevoltage Vrs between the paths R, S, a line voltage Vst between the pathsS, T, and a line voltage Vrt between the paths R, T in a non-contactmanner.

As shown in FIG. 21, the respective voltage detecting apparatuses 201are constructed in the same way with the floating circuit unit 2D andthe main circuit unit 3C described above and, with the paths R, S, T asthe detected objects, detect the effective values of the AC voltagesVrp, Vsp, Vtp (the detected AC voltages) and output data showing theeffective values as detection data Dva, Dvb, Dvc. Hereinafter, when nospecial distinction is required, the voltage data Dva, Dvb, Dvc arecollectively referred to as the “detection data Dv”. In the presentembodiment, the output unit 235 of each voltage detecting apparatus 201is constructed of a transmission apparatus that is capable of datatransmission and equipped with a function that transmits the detectiondata Dva, Dvb, Dvc inputted from the processing unit 233 to thecalculation unit 252. Note that since the other component elements asidefrom the output unit 235 of the voltage detecting apparatus 201 are thesame as in the constructions described earlier, detailed descriptionthereof is omitted.

The calculation unit 252 includes a CPU and a memory (neither of whichis shown), and carries out a line voltage calculating process thatcalculates (detects) the line voltages based on the detection data Dvoutputted from the respective voltage detecting apparatuses 201. Thecalculation unit 252 displays the result of the line voltage calculatingprocess on the display unit 253. In the present embodiment, the displayunit 253 is constructed of a monitor apparatus such as a liquid crystaldisplay. Note that it is also possible to construct the display unit 253of a print apparatus such as a printer. Also, as described later, theparts (for example, the respective cases of the main circuit units 3C)G1 of the main circuit units 3C of the respective voltage detectingapparatuses 201 that are used as the ground potential Vg are connectedtogether. Also, as one example, the calculation unit 252 and the displayunit 253 operate by being supplied with a voltage from a power supplycircuit (not shown) included in one main circuit unit 3C out of thethree main circuit units 3C.

Next, a detection operation by the line voltage detecting apparatus 251will be described.

First, during detection, as shown in FIG. 21, to detect the AC voltageVrp of the path R using the voltage detecting apparatus 201 r, thefloating circuit unit 2D of the voltage detecting apparatus 201 r ispositioned near the path R and the detection electrode 212 of thevoltage detecting apparatus 201 r is positioned opposite thecorresponding path R. In the same way, to detect the AC voltages Vsp,Vtp of the paths S, T, the detection electrodes 212 of the respectivefloating circuit units 2D of the voltage detecting apparatuses 201 s,201 t are positioned opposite the corresponding paths S, T. By doing so,the capacitance C0 (see FIG. 13) is formed between each of therespective detection electrodes 212 and the paths R, S, T and thevoltage detecting apparatuses 201 r, 201 s, 201 t start detecting the ACvoltages Vrp, Vsp, Vtp of the corresponding paths R, S, T. Here, asdescribed earlier, in the voltage detecting apparatuses 201 r, 201 s,201 t, the AC voltages Vrp, Vsp, Vtp are accurately detected by theprocessing units 233 regardless of the magnitudes of the capacitancesC0.

Here, in the respective voltage detecting apparatuses 201 r, 201 s, 201t, the output units 235 output the effective values of the AC voltagesVrp, Vsp, Vtp of the paths R, S, T calculated by the respectiveprocessing units 233 as the detection data Dva, Dvb, Dvc.

The calculation unit 252 inputs the detection data Dva, Dvb, Dvcoutputted from the respective voltage detecting apparatuses 201 andstores the detection data Dva, Dvb, Dvc in the memory. Next, thecalculation unit 252 carries out the line voltage calculating process.More specifically, the calculation unit 252 calculates the voltagedifference between the effective values of the AC voltages Vrp, Vspshown by the detection data Dva, Dvb to find (detect) the line voltageVrs between the paths R, S. In the same way, the calculation unit 252calculates the voltage difference between the effective values of the ACvoltages Vsp, Vtp shown by the detection data Dvb, Dvc to find (detect)the line voltage Vst between the paths S, T and calculates the voltagedifference between the effective values of the AC voltages Vrp, Vtpshown by the voltage data Dva, Dvc to find (detect) the line voltage Vrtbetween the paths R, T. The calculation unit 252 also has the calculatedline voltages Vrs, Vst, Vrt displayed on the display unit 253.

In this way, according to the line voltage detecting apparatus 251, byusing the voltage detecting apparatuses 201, even if the coupledcapacitances (the capacitances C0) between the detection electrodes 212of the respective voltage detecting apparatuses 201 and the respectivepaths R, S, T as the detected objects for the voltage detectingapparatuses 201 are unknown, it will still be possible to accuratelydetect the line voltages Vrs, Vst, Vrt across a wide frequency band in anon-contact manner without calculating the coupled capacitances.

It is also possible to construct the voltage detecting apparatus 201 soas to be equipped with a self-diagnosis function that detects the levelof the signal component of the standard signal Ss (a signal componentwith the same frequency as the standard signal Ss) included in onesignal out of the insulated detection signal S2 and the amplifieddetection signal S3 and judges (diagnoses) whether the apparatus isoperating normally. More specifically, for an example where theinsulated detection signal S2 is used, as shown by the broken line inFIG. 13, a filter (band pass filter) 238 that passes a signal componentwith the same frequency as the standard signal Ss is provided and byinputting the insulated detection signal S2 detected at the point A intothe filter 238, the signal component of the standard signal Ss includedin the insulated detection signal S2 is extracted and outputted to theprocessing unit 233. The processing unit 233 functions as a judgingunit, and converts the signal component of the inputted standard signalSs to digital data, detects the level (the amplitude level of the signalcomponent or a DC voltage level (an absolute value of a DC voltage)after rectification of such signal component) and compares the levelwith a set level set in advance to carry out self-diagnosis.

Here, in the voltage detecting apparatus 201 when the detectionelectrode 212 and the detected object 4 are capacitively coupled in astate where the apparatus is operating normally, the current signal I(more specifically, the standard current component Is1) due to thestandard signal Ss outputted from the standard signal outputting unit231 will flow between the detected object 4 and the floating circuitunit 2D and a signal component due to the standard current component Is1will always be included in the insulated detection signal S2 and theamplified detection signal S3. Accordingly, by setting the level of thestandard signal Ss included in the insulated detection signal S2 in thisstate as the set level in advance, self-diagnosis on the voltagedetecting apparatus 201 can be realized by the processing unit 233carrying out at least one process out of a judging process that judgesthat the apparatus is operating normally (a state called “normaloperation”) when the detected level of the signal component of thestandard signal Ss is equal to or greater than the set level and ajudging process that judges that the apparatus is operating abnormally(a state called “abnormal operation”) when the detected level of thesignal component of the standard signal Ss is below the set level. It isalso possible to use a construction where the processing unit 233outputs the result of the self-diagnosis to the output unit 235 and bydoing so, it is possible to inform the outside whether the apparatus isoperating normally. It is also possible to use a construction where theprocessing unit 233 stores the result of the self-diagnosis in thestorage unit 234 so that by confirming the stored result of theself-diagnosis, it is possible to detect whether the apparatus isoperating normally (or in other words, whether an error has occurred).It is also possible to use a construction where the amplified detectionsignal S3 detected at the point B is inputted into the filter 238 inplace of the insulated detection signal S2, and with such constructionalso, it is possible to realize self-diagnosis in the same way as in theconstruction described above that uses the insulated detection signalS2.

As a construction for carrying out self-diagnosis, it is also possiblefor the voltage detecting apparatus 201 to use a construction thatdetects one level out of a level of the signal component of the standardsignal Ss included in the output signal So (the amplitude level of suchsignal component or a DC voltage level (an absolute value of a DCvoltage) after rectification of such signal component) and a level ofthe wave detection signal Vd (an absolute value of a DC voltage) andcarries out self diagnosis based on the detected level. Morespecifically, when the output signal So is used as one example, in placeof the construction shown by the broken line in FIG. 13 (i.e., aconstruction where the insulated detection signal S2 is inputted intothe filter 238), a construction is used where the output signal Sodetected at the point C is inputted into the filter 238, and the signalcomponent of the standard signal Ss included in the output signal So isextracted and outputted to the processing unit 233. The processing unit233 functions as a judging unit, and converts the signal component ofthe inputted standard signal Ss to digital data, detects the level (theamplitude level or the DC voltage level) and compares the level with aset level set in advance to carry out self-diagnosis.

Here, when the voltage detecting apparatus 201 is operating normally,since the adder circuit 242 adds the amplified detection signal S3 andthe reference signal Sr1 so that the signal components of the standardsignal Ss included in the signals S3, Sr1 cancel each other out, theamplitude of the signal component of the standard signal Ss included inthe output signal So will have an extremely low level. Accordingly, bysetting in advance an upper limit value (a permitted maximum level) forthe level (amplitude level or DC voltage level described above) of thesignal component of the standard signal Ss included in the output signalSo as a set level, self-diagnosis on the voltage detecting apparatus 201can be realized by the processing unit 233 carrying out at least oneprocess out of a judging process that judges that the apparatus isoperating normally (a state called “normal operation”) when the detectedlevel of the signal component of the standard signal Ss is equal to orless than the set level and a judging process that judges that theapparatus is operating abnormally (a state called “abnormal operation”)when the detected level of the signal component of the standard signalSs is above the set level. It is also possible to use a constructionwhere the processing unit 233 outputs the result of the self-diagnosisto the output unit 235 and by doing so, it is possible to inform theoutside whether the apparatus is operating normally. It is also possibleto use a construction where the processing unit 233 stores the result ofthe self-diagnosis in the storage unit 234 so that by confirming thestored result of the self-diagnosis, it is possible to detect whetherthe apparatus is operating normally (or in other words, whether an errorhas occurred). Also, in place of a construction that detects the levelof the signal component of the standard signal Ss included in the outputsignal So, it is also possible to use a construction that uses the levelof the wave detection signal Vd detected at the point D. With thisconstruction, since the level of the wave detection signal Vd is a DCcomponent generated due to only the signal component of the standardsignal Ss, it is possible to omit the filter 238. With this constructionalso, in the same way as the above construction that uses the level ofthe signal component of the standard signal Ss included in the outputsignal So, by comparing the level (an absolute value of a DC voltage) ofthe wave detection signal Vd to a set level set in advance for suchlevel, it is possible to carry out self-diagnosis.

Also, although a construction has been described where the processingunit 233 compares a signal component of the standard signal Ss extractedfrom one signal out of the insulated detection signal S2, the amplifieddetection signal S3, and the output signal So, or the result ofconverting the wave detection signal Vd to digital data with a setlevel, when a construction is used where the extracted signal componentof the standard signal Ss is rectified and converted to a DC voltage, itwill also be possible to use a comparator to carry out the comparisonwith the set level. In the same way for the wave detection signal Vdalso, it should be obvious that it is possible to use a constructionwhere a comparator is used to carry out the comparison with the setlevel. Also, although the construction is simplified when the filter 238is used to extract the signal component of the standard signal Ss fromthe insulated detection signal S2 or the amplified detection signal S3,it is also possible to use a construction that extracts the signalcomponent by carrying out synchronous detection using the standardsignal Ss.

1. A voltage detecting apparatus that detects a detected AC voltagegenerated in a detected object, comprising: a detection electrode thatis disposed facing the detected object; a current-to-voltage convertingcircuit including an operational amplifier that has a first inputterminal set at a reference voltage and a second input terminal directlyor indirectly connected to the detection electrode and that converts adetection current, which flows with a current value that corresponds toan AC potential difference between the detected AC voltage and thereference voltage on a path that includes the detection electrode and afeedback circuit connected to the second input terminal, to a detectionvoltage signal and outputs the detection voltage signal; an integratingcircuit that integrates the detection voltage signal and outputs anintegrated signal whose amplitude changes in accordance with thepotential difference; an insulating circuit that is disposed before orafter the integrating circuit, inputs detection voltage signal andoutputs the detection voltage signal so as to be electrically insulatedfrom an input side thereof when disposed before the integrating circuit,and inputs the integrated signal and outputs the integrated signal so asto be electrically insulated from an input side thereof when disposedafter the integrating circuit; and a voltage generating circuit thatamplifies a signal based on the integrated signal so as to reduce thepotential difference and thereby generates the reference signal.
 2. Thevoltage detecting apparatus according to claim 1, wherein the insulatingcircuit includes an optical insulating element and/or a transformer, andoutputs the integrated signal, which is an analog signal, so as to beelectrically insulated from the input side thereof.
 3. The voltagedetecting apparatus according to claim 1, wherein the insulating circuitincludes a digital isolator, and outputs the integrated signal, which isa digital signal, so as to be electrically insulated from the input sidethereof.
 4. The voltage detecting apparatus according to claim 1,further comprising a guard electrode that is set at the referencevoltage and covers circuits from a circuit that comes after thedetection electrode to a primary-side circuit of the insulating circuit.5. The voltage detecting apparatus according to claim 4, wherein anopening is formed in the guard electrode and the detection electrode isdisposed at a position inside the guard electrode so as to face theopening in a state where the detection electrode does not protrude fromthe opening.
 6. The voltage detecting apparatus according to claim 1,further comprising an insulator that covers an entire surface of thedetection electrode that faces the detected object.
 7. A line voltagedetecting apparatus comprising: a plurality of voltage detectingapparatuses according to claim 1 that are constructed so that thedetection electrodes thereof are capable of being disposed facing aplurality of paths that correspond to the detected objects thereof andso as to be capable of detecting AC voltages generated on the respectivepaths as the detected AC voltages thereof; and a calculation unit thatcalculates a voltage difference between the AC voltages on two of thepaths detected by a pair of voltage detecting apparatuses out of theplurality of voltage detecting apparatuses and thereby finds a linevoltage between the two paths.
 8. A voltage detecting apparatus thatdetects a detected AC voltage generated in a detected object,comprising: a detection electrode that is disposed facing the detectedobject; a current-to-voltage converting circuit including a detectionunit that is disposed between the detection electrode and a position ofa reference voltage and converts a detection current, which flows with acurrent value that corresponds to an AC potential difference between thedetected AC voltage and the reference voltage, to a voltage signal andan amplifier that converts an impedance of the voltage signal andoutputs the converted voltage signal as a detection voltage signal; anintegrating circuit that integrates the detection voltage signal andoutputs an integrated signal whose amplitude changes in accordance withthe potential difference; an insulating circuit that is disposed beforeor after the integrating circuit, inputs the detection voltage signaland outputs the detection voltage signal so as to be electricallyinsulated from an input side thereof when disposed before theintegrating circuit, and inputs the integrated signal and outputs theintegrated signal so as to be electrically insulated from an input sidethereof when disposed after the integrating circuit; and a voltagegenerating circuit that amplifies a signal based on the integratedsignal so as to reduce the potential difference and thereby generatesthe reference signal.
 9. The voltage detecting apparatus according toclaim 8, wherein the insulating circuit includes an optical insulatingelement and/or a transformer, and outputs the integrated signal, whichis an analog signal, so as to be electrically insulated from the inputside thereof.
 10. The voltage detecting apparatus according to claim 8,wherein the insulating circuit includes a digital isolator, and outputsthe integrated signal, which is a digital signal, so as to beelectrically insulated from the input side thereof.
 11. The voltagedetecting apparatus according to claim 8, further comprising a guardelectrode that is set at the reference voltage and covers circuits froma circuit that comes after the detection electrode to a primary-sidecircuit of the insulating circuit.
 12. The voltage detecting apparatusaccording to claim 11, wherein an opening is formed in the guardelectrode and the detection electrode is disposed at a position insidethe guard electrode so as to face the opening in a state where thedetection electrode does not protrude from the opening.
 13. The voltagedetecting apparatus according to claim 8, further comprising aninsulator that covers an entire surface of the detection electrode thatfaces the detected object.
 14. A line voltage detecting apparatuscomprising: a plurality of voltage detecting apparatuses according toclaim 8 that are constructed so that the detection electrodes thereofare capable of being disposed facing a plurality of paths thatcorrespond to the detected objects thereof and so as to be capable ofdetecting AC voltages generated on the respective paths as the detectedAC voltages thereof; and a calculation unit that calculates a voltagedifference between the AC voltages on two of the paths detected by apair of voltage detecting apparatuses out of the plurality of voltagedetecting apparatuses and thereby finds a line voltage between the twopaths.